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THE  DEVELOPMENT  OF 
THE  HUMAN  BODY 


MCMURRICH 


MORRIS'S  ANATOMY 

FIFTH  EDITION 

UNDER  AMERICAN  EDITORSHIP 

Rewritten,  Revised,  Improved,  with  Many  New  Illustralions 

EDITED  BY 

C.  M.  JACKSON,  M.  S.,  M.  D. 

Professor  and  Director  of  the  Department  of  Anatomy 
University  of  Minnesota 

Among  the  American  contributors  will  be  noted:  C.  M.  Jackson, 
J.  Playfair  McMurrich,  R.  J.  Terry,  Irving  Hardesty,  Abram  T. 
Kerr,  Charles  R.  Bardeen,  Eliot  R.  Clark,  and  H.  D.  Senior. 
F.  W.  Jones,  John  Morley,  Peter  Thomson,  David  Waterston 
head  the  English  contributors. 

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Medical  Record,  New  York. 

The  text  has  been  completely  revised.  Very  special  attention,  in  this  new 
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It  contains  many  features  of  special  advantage  to  students.  It  is  modern,  up  to 
date  in  every  respect.  It  has  been  carefully  revised,  and  in  many  parts  rewritten, 
and  includes  many  new  features. 

Containing  1182  Illustrations,  of  which  358  are  in  colors. 

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PART     v.— Clinical  and  Topographical  .A.natomy.     Index.     $2.25. 


THE  DEVELOPMENT  OF 
THE  HUMAN  BODY 


A  MANUAL 
OF  HUMAN  EMBRYOLOGY 


J.  PLAYFAIR  iyicMURRICH,  A.  M.,  Ph.  D.,  LL.  D. 

PROFESSOR  OF  ANATOMY   IN   THE   UNIVERSITY  OF  TORONTO 
FORMERLY   PROFESSOR  OF   ANATOMY   IN  THE   UNIVERSITY   OF   MICHIGAN 


SIXTH  EDITION,  REVISED  AND  ENLARGED 


With  Two  Hundred  and  Ninety  Illustrations  Several 
of  which  are  Printed  in  Colors 


PHiLADiLf^HlA 

P.  RLA)CLSTON'S  SON  &  CO 
^  ^  ^    ;  ibii  Walnut  31 REET 


Copyright,  1920,  by  P.  Blakiston's  Son  &  Co. 


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PREFACE  TO  THE  SIXTH  EDITION 


The  increasing  interest  in  human  and  mammalian  embryology 
which  has  characterized  the  last  few  years  has  resulted  in  many 
additions  to  our  knowledge  of  these  branches  of  science,  and  has 
necessitated  not  a  few  corrections  of  ideas  formerly  held.  In  this 
sixth  edition  of  this  book,  as  in  previous  ones,  the  attempt  has 
been  made  to  incorporate  the  results  of  all  important  recent  contri- 
butions upon  the  topics  discussed,  and.  at  the  same  time,  to  avoid 
any  considerable  increase  in  the  bulk  of  the  volume.  Several 
chapters  have,  therefore,  been  largely  recast,  and  the  subject 
matter  has  been  thoroughly  revised  throughout,  so  that  it  is 
hoped  that  the  book  forms  an  accurate  statement  of  our  present 
knowledge  of  the  development  of  the  human  body. 

In  addition  to  the  works  mentioned  in  the  preface  to  the  first 
edition  as  of  special  value  to  the  student  of  Embryology,  mention 
should  be  made  of  the  Handhuch  der  vergleichenden  und  experimen- 
tellen  Entwicklungslehre  der  Wirheltiere  edited  by  Professor  Oscar 
Hertwig  and  especially  of  the  Manual  of  Human  Embryology 
edited  by  Professors  F.  Keibel  and  F.  P.  Mall. 
University  of  Toronto. 


81158 


PREFACE  TO  THE  FIRST  EDIIION 


The  assimilation  of  the  enormous  mass  of  facts  which  consti- 
tute what  is  usually  known  as  descriptive  anatomy  has  always 
been  a  difficult  task  for  the  student.  Part  of  the  difficulty  has 
been  due  to  a  lack  of  information  regarding  the  causes  which  have 
determined  the  structure  and  relations  of  the  parts  of  the  body,  for 
without  some  knowledge  of  the  why  things  are  so,  the  facts  of 
anatomy  stand  as  so  many  isolated  items,  while  with  such  knowl- 
edge they  become  bound  together  to  a  continuous  whole  and  their 
study  assumes  the  dignity,  of  a  science. 

The  great  key  to  the  significance  of  the  structure  and  relations 
of  organs  is  their  development,  recognizing  by  that  term  the 
historical  as  well  as  the  individual  development,  and  the  following 
pages  constitute  an  attempt  to  present  a  concise  statement  of  the 
development  of  the  human  body  and  a  foundation  for  the  proper 
understanding  of  the  facts  of  anatomy.  Naturally,  the  individual 
development  claims  the  major  share  of  attention,  since  its  pro- 
cesses are  the  more  immediate  forces  at  work  in  determining  the 
conditions  in  the  adult,  but  where  the  embryological  record  fails  to 
afford  the  required  data,  whether  from  its  actual  imperfection  or 
from  the  incompleteness  of  our  knowledge  concerning  it,  recourse 
has  been  had  to  the  facts  of  comparative  anatomy  as  affording 
indications  of  the  historical  development  or  evolution  of  the  parts 
under  consideration. 

It  has  not  seemed  feasible  to  include  in  the  book  a  complete  list 
of  the  authorities  consulted  in  its  preparation.  The  short 
bibliographies  appended  to  each  chapter  make  no  pretensions 
to  completeness,  but  are  merely  indications  of  some  of  the  more 
important  works,  especially  those  of  recent  date,  which  con- 
sider the  questions  discussed.  For  a  very  full  bibliography  of 
all  works  treating  of  human  embryology  up  to   1893  reference 

vii 


Vlll  PREFACE   TO   THE   FIRST   EDITION 

may  be  made  to  Minot's  Bibliography  of  Vertebrate  Embryology, 
published  in  the  "Memoirs  of  the  Boston  Society  of  Natural 
History,"  volume  iv,  1893.  It  is  fitting,  however,  to  acknowledge 
an  especial  indebtedness,  shared  by  all  writers  on  human 
embryology,  to  the  classic  papers  of  His,  chief  among  which  is 
his  Anatomie  menschlicher  Embryonen,  and  grateful  acknowledge- 
ments are  also  due  to  the  admirable  text-books  of  Minot,  O. 
Hertwig,  and  Kollmann. 

Anatomical  Laboratory, 
University  of  Michigan. 


CONTENTS 

Paob 
rXRODUCTION I 

PART  I.— GENERAL  DEVELOPMENT 
CHAPTER  I 

The  Spermatozoon  and  Spermatogenesis;  the  Ovum  and  Its  Matu- 
ration and  Fertilization ii 

CHAPTER  II 

The  Segmentation  of  the  Ovum  and  the  Formation  of  the  Germ 

Layers 41 

CHAPTER  III 

The  Medullary  Groove,  Notochord,  and  Mesodermic  Somites  .    .     67 

CHAPTER  IV 
The  Development  of  the  External  Form  of  the  Human  Embryo       .89 

CHAPTER  V 
The  Yolk-stalk,  Belly-stalk,  and  Fetal  Membranes      no 

PART  II.— ORGANOGENY 
CHAPTER  VI 

The  Development  of  the  Integumentary  System 143 

CHAPTER  VII 

The  Development  of  the  Connective  Tissues  and  Skeleton     .    .    .   155 

CHAPTER  VIII 

The  Development  of  the  Muscular  System 195 

is 


X 


CONTENTS 


CHAPTER  IX 

The  Development  of  the  Circulatory  and  Lymphatic  Systems.    .   222 

CHAPTER  X 
The  Development  of  the  Digestive  Tract  and  Glands 282 

CHAPTER  XI 

The  Development  of  the  Pericardium,  the  Pleuro-peritoneum,  and 

the  Diaphragm 3^9 

CHAPTER  XII 
The  Development  of  the  Organs  of  Respiration 334 

CHAPTER  XIII 
The  Development  of  the  Urinogenital  System 341 

CHAPTER  XIV 
The  Suprarenal  System  of  Organs 374 

CHAPTER  XV 
The  Development  of  the  Nervous  System 381 

CHAPTER  XVI 

The  Development  of  the  Organs  of  Special  Sense 432 

CHAPTER  XVII 

Post-natal  Development 475 

Index 491 


THE  DEVELOPMENT 

OF  THE 

HUMAN  BODY 


INTRODUCTION 

One  of  the  fundamental  principles  of  biology  is  that  which 
regards  all  organisms  as  composed  of  one  or  more  structural  units, 
termed  cells.  Each  of  these  maintains  an  individual  existence  and 
in  multicellular  organism  is  influenced  by  its  fellows  and  contri- 
butes with  them  to  the  maintenance  of  the  general  existence  of  the 
individual  of  which  it  is  a  part.  This  is  the  cell  theory  formulated 
by  Schleiden  and  Schwann  (1839),  and  according'to  it  the  human 
body,  though  physiologically  a  unit,  is,  structurally,  a  community, 
an  aggregate  of  many  individual  units,  each  of  which  leads  to 
a  certain  extent  an  independent  existence  and  yet  both  contributes 
to  and  shares  in  the  general  welfare  of  the  community. 

To  the  founders  of  the  theory  the  structural  units  were  vesicles 
with  definite  walls,  and  little  attention  was  paid  to  their  contents. 
Hence  the  use  of  the  term  "cell"  in  connection  with  them.  Long 
before  the  establishment  of  the  cell  theory,  however,  the  existence 
of  organisms  composed  of  a  gelatinous  substance  showing  no  indi- 
cations of  a  definite  limiting  membrane  had  been  noted,  and  in 

5  a  French  naturalist,  Dujardin,  had  described  the  gelatinous 
material  of  which  certain  marine  organisms  (Rhizopoda)  are 
composed,  terming  it  sarcode  and  maintaining  it  to  be  the  material 
substratum  which  conditioned  the  various  vital  phenomena  exhib- 

Kd  by  the  organisms.     Later,  in  1846,  a  botanist,  von  Mohl, 


2  INTRODUCTION 

observed  that  living  plant  cells  contained  a  similar  substance,  upon 
which  he  believed  the  existence  of  the  cell  as  a  vital  structure  was 
dependent,  and  he  bestowed  upon  this  substance  the  name  proto- 
plasm, by  which  it  is  now  universally  known. 

By  these  discoveries  the  importance  originally  attributed  to  the 
cell- wall  was  greatly  lessened,  and  in  1864  Max  Schultze  reformu- 
lated the  cell  theory,  defining  the  cell  as  a  mass  of  protoplasm,  the 
presence  or  absence  of  a  limiting  membrane  or  cell-wall  being  im- 
material. At  the  same  time  the  spontaneous  origination  of  cells 
from  an  undifferentiated  matrix,  believed  to  occur  by  the  older 
authors,  was  shown  to  have  no  existence,  every  cell  originating  by 
the  division  of  a  preexisting  cell,  a  fact  concisely  expressed  in  the 
aphorism  of  Virchow — omnis  cellula  a  celluld. 

Interpreted  in  the  light  of  these  results,  the  human  body  is  an 
aggregate  of  myriads  of  cells* — i.e.,  of  masses  of  protoplasm,  each 
of  which  owes  its  origin  to  the  division  of  a  preexistent  cell  and  all 
of  which  may  be  traced  back  to  a  single  parent  cell — a  fertilized 
ovum.  All  these  cells  are  not  alike,  however,  but  just  as  in  a 
social  community  one  group  of  individuals  devotes  itself  to  the 
performance  of  one  of  the  duties  requisite  to  the  well-being  of 
the  community  and  another  group  devotes  itself  to  the  perform- 
ance of  another  duty,  so  too,  in  the  body,  one  group  of  cells  takes 
upon  itself  one  special  function  and  another  another.  There  is, 
in  other  words,  in  the  cell-community  a  physiological  division  of 
labor.  Thus  certain  cells  become  especially  contractile,  forrring 
muscle  cells;  others  become  especially  irritable,  responding  readily 
to  stimulation,  and  form  nerve  cells;  others  undertake  the  forma- 
tion of  this  or  that  secretion  useful  to  the  organism  as  a  whole  and 
are  gland  cells ;  while  others  set  themselves  apart  for  the  reproduc- 
tion of  the  species.  Each  functional  specialization  is  associated 
with  a  more  or  less  definite  structural  adaptation,  so  that  the 
general  function  of  a  cell  may  be  recognized  from  its  form  and 
structure.     The  comparison  of  the  cell-community  to  the  social 

*  It  has  been  estimated  that  the  number  of  cells  entering  into  the  composition  of 
the  body  of  an  adult  human  being  is  about  twenty-six  million  five  hundred  thousand 
millions. 


INTRODUCTION  3 

community  may  thus  be  carried  still  further,  for  just  as  gradations 
of  individuality  maybe  recognized  in  the  individual,  the  municipal- 
ity, and  the  state,  so  too  in  the  cell-community  there  are  cells; 
tissues,  each  of  which  is  an  aggregate  of  similar  cells;  organs^ 
which  are  aggregates  of  tissues,  one,  however,  predominating  and 
determining  the  character  of  the  organ;  and  systems,  which  are 
aggregates  of  organs  having  correlated  functions. 

It  is  the  province  of  embryology  to  study  the  mode  of  division 
of  the  fertilized  ovum  and  the  progressive  differentiation  of  the  re- 
sulting cells  to  form  the  tissues,  organs,  and  systems.  But  before 
considering  these  phenomena  as  seen  in  the  human  body  it  will  be 
well  to  get  some  general  idea  of  the  structure  of  an  animal  cell. 
This  as  has  been  already  stated,  is  a  mass  of  protoplasm,  but, 
as  a  rule,  one  finds  imbedded  in  this  various  products  of  its  activi- 
ties, such  as  globules  of  fat,  pigment,  or  secretion  granules,  all 
of  which  may  be  grouped  together  as  deutoplasm  (Fig.  i).  The 
protoplasm  itself  is  a  viscous  substance  resembling  egg-albumen 
in  many  of  its  physical  peculiarities  and  like  this  being  coagulated 
by  heat  or  when  it  is  exposed  to  the  action  of  various  chemical 
reagents.  It  is  to  be  regarded  as  a  colloidal  mixture,  whose 
principal  constituents  are  albuminous  and  lipoid  susbtances  in 
varying  proportions,  the  term  protoplasm  not  connoting  any  defi- 
nite chemical  compound,  but  being  rather  a  morphological  con- 
cept denoting  all  those  colloidal  complexes  whose  activities  result 
in  the  manifestation  of  the  phenomena  which  we  term  Life. 

The  protoplasm  of  an  animal  cell  is,  however,  by  no  means  a 
homogeneous  material.  Even  in  the  living  cell  what  is  termed  a 
nucleus  (Fig.  i.  A'')  is  usually  clearly  discernible  as  a  more  or  less 
spherical  body  of  a  greater  refractive  index  than  the  surround- 
ing protoplasm,  and  since  this  is  a  permanent  organ  of  the  cell  it  is 
convenient  to  distinguish .  the  surrounding  protoplasm  as  cyto- 
plasm from  the  nuclear  protoplasm  or  karyo plasm.  But  the 
structure  of  the  nucleus  and  other  organs  of  the  cell  can  be  more 
accurately  determined  when  the  protoplasm  has  been  ''fixed" 
or  coagulated  by  certain  reagents  and  then  subjected  to  the 
action  of  dyes  which  have  a  selective  affinity  for  the  various  struc- 


4  INTRODUCTION 

tural  constituents.  Treated  thus  both  cytoplasm  and  karyoplasm 
present  the  appearance  of  a  more  solid  reticulum  forming  a  net- 
work in  whose  meshes  is  a  more  fluid  material,  the  enchylema.  At 
the  surface  of  the  cell  the  cytoplasmic  reticulum  passes  over  in  o  a 
more  homogeneous  layer,  which  may  be  distinguished  even  in  the 
living  cell  by  its  greater  firmness  and  resistence  as  compared  with 
the  more  fluid  central  material.  This  surface  pellicle  is  termed  the 
ectoplasm  (Fig.  i ,  Ect)  as  distinguished  from  the  central  endoplasm 
{End)  and  there  is  a  similar  pellicle  enclosing  the  karyoplasm. 


Pig.  I. — Diagram  Showing  the  Structure  of  a  Cell.  Ar,  Archoplasm 
Sphere;  cho,  Chondriosome;  chr.  Chromatin;  Dp,  Deutoplasm;  Ect,  Ecto- 
plasm; End,  Endoplasm;  N,  Nucleus;  n.  Nucleolus. 

forming  the  nuclear  membrane.  In  addition  to  the  reticulum  and 
enchylema  the  karyoplasm  has  scattered  along  the  fibres  of  its 
reticulum  a  peculiar  material  termed  chromatin  and  usually  con- 
tains, embedded  in  its  substance ,  one  or  more  spherical  bodies 
termed  nucleoli,  which  may  be  merely  larger  masses  of  chromatin  or 
bodies  of  special  chemical  composition.  Further,  in  all  actively 
growing  cells  there  is  differentiated  in  the  cytoplasm  a  peculiar 
body  known  as  the  archoplasm  sphere  (Fig.  i,  Ar),  in  the  center  of 
which  there  is  usually  a  minute  spherical  body,  known  as  the 
centrosome,  these  structures  playing  an  important  part  in  the  repro- 
duction of  the  cell  by  division.     Finally  there  are  also  present 


INTRODUCTION  5 

in  the  cytoplasm  structures  termed  chondriosomes  (Fig.  i,  Cho) 
which  have  the  form  of  minute  granules,  mitochondria,  or  rods, 
chondrioconts,  and  have  been  supposed  by  some  observers  to  be^ 
concerned  with  the  formation  of  special  products  of  the  cytoplasm, 
such  as  neurofibrils,  secretion  products,  etc. 

It  has  been  already  stated  that  new  cells  arise  by  the  division  of 
preexisting  ones,  and  this  process  is  associated  with  a  series  of  com- 
plicated phenomena  which  have  great  significance  in  connection 
with  some  of  the  problems  of  embryology.  When  such  a  cell 
as  has  been  described  above  is  about  to  divide,  the  fibers  of  the 
reticulum  in  the  neighborhood  of  the  archoplasm  sphere  arrange 
themselves  so  as  to  form  fibrils  radiating  in  all  directions  from  the 
sphere  as  a  center,  and  the  archoplasm  with  its  contained  centro- 
some  gradually  elongates  and  finally  divides,  each  portion  retain- 
ing its  share  of  the  radiating  fibrils,  so  that  two  asters,  as  the  aggre- 
gate of  centrosome,  sphere  and  fibrils  is  termed,  are  now  to  be 
found  in  the  cytoplasm  (Fig.  2,  A).  Gradually  the  two  asters 
separate  from  one  another  and  eventually  come  to  rest  at  opposite 
sides  of  the  nucleus  (Fig.  2,  C).  In  this  structure  important 
changes  have  been  taking  place  in  the  meantime.  The  nuclear 
membrane  disappears  and  the  chromatin,  originally  scattered 
irregularly  along  the  reticulum,  gradually  aggregates  to  form  a 
continuous  thread  (Fig.  2,  A)  and  later  this  thread  breaks  up  into 
a  definite  number  of  pieces,  termed  chromosomes  (Fig.  2,  B),  the 
number  of  these  being  practically  constant  for  each  species  of 
animal.  The  number  occurring  in  man  is  probably  twenty-four 
(Flemming,  Duesberg,  Wieman). 

As  soon  as  the  asters  have  taken  up  their  position  on  opposite 
sides  of  the  nucleus,  the  nuclear  reticulum  begins  to  be  converted 
into  a  spindle-shaped  bundle  of  fibrils  which  associate  themselves 
with  the  astral  rays  and  have  lying  scattered  among  them  the 
chromosomes  (Fig.  2,  C).  To  the  figure  so  formed  the  term 
amphiaster  is  applied,  and  soon  after  its  formation  the  chromo- 
somes arrange  themselves  in  a  circle  or  plane  at  the  equator  of 
the  spindle  (Fig.  2,  D)  and  the  stages  preparatory  to  the  actual 
division,  the  prophases,  are  completed. 


0  INTRODUCTION 

The  next  stage,  the  metaphase  (Fig.  3,  ^),  consists  of  the  divi- 
sion, usually  longitudinally,  of  each  chromosome,  so  that  the  cell 
now  contains  twice  as  many  chromosomes  as  it  did  previously.  As 
soon  as  this  division  is  completed  the  anaphases  are  inaugurated 
by  the  halves  of  each  chromosome  separating  from  one  another  and 
approaching  one  of  the  asters  (Fig.  3,  B),  and  a  group  of  chromo- 


FlG.     2. 


-Diagrams  Illustrating  the  Prophases  of   Mitosis. — (Adapted  from 
E.  B.  Wilson.) 


somes,  containing  half  the  total  number  formed  in  the  metaphase, 
comes  to  lie  in  close  proximity  to  each  archoplasm  sphere  (Fig.  3, 
C).  The  spindle  and  astral  fibers  gradually  resolve  themselves 
again  into  the  reticulum  and  the  chromosomes  of  each  group 
become  irregular  in  shape  and  gradually  spread  out  upon  the 
nuclear  reticulum  so  that  two  nuclei,  each  similar  to  the  one  from 


INTRODUCTION 


which  the  process  started,  are  formed  (Fig.  3,  Z)).  Before  all 
these  changes  are  accomplished,  however,  a  constriction  makes 
its  appearance  at  the  surface  of  the  cytoplasm  (Fig.  3,  C)  and, 
gradually  def^pening,  divides  the  cytoplasm  in  a  plane  passing 
through  the  equator  of  the  amphiaster  and  gives  rise  to  two 
separate  cells  (Fig.  3,  Z>). 


Fig.  3. — Diagrams  Illustrating  the  Metaphase  and  Anaphases  of  Mitosis. — 
^Adapted  from  E.  B.  Wilson.) 


This  complicated  process,  which  is  known  as  karyokinesis  or 
mitosis,  is  the  one  usually  observed  in  dividing  cells,  but  occasion- 
ally a  cell  divides  by  the  nucleus  becoming  constricted  and  divid- 
ing into  two  parts  without  any  development  of  chromosomes, 
spindle,  etc.,  the  division  of  the  cell  following  that  of  the  nucleus.^ 
This  amitotic  method  of  division  is,  however,  rare,  and  in  many 


8  INTRODUCTION 

cases,  though  not  always,  its  occurrence  seems  to  be  associated  with 
an  impairment  of  the  reproductive  activities  of  the  cells.  In 
actively  reproducing  cells  the  mitotic  method  of  division  may  be 
regarded  as  the  rule. 

Since  the  process  of  development  consists  of  the  multiplication 
of  a  single  original  cell  and  the  differentiation  of  the  cell  aggregate 
so  formed,  it  follows  that  the  starting-point  of  each  line  of  indi- 
vidual development  is  to  be  found  in  a  cell  which  forms  part  of  an 
individual  of  the  preceding  generation.  In  other  words,  each  in- 
dividual represents  one  generation  in  esse  and  the  succeeding  gene- 
ration in  posse.  This  idea  may  perhaps  be  made  clear  by  the 
following  considerations.  As  a  result  of  the  division  of  a  fertilized 
ovum  there  is  produced  an  aggregate  of  cells,  which,  by  the  physio- 
logical division  of  labor,  specialize  themselves  for  various  func- 
tions. Some  assume  the  duty  of  perpetuating  the  species  and  are 
known  as  the  sexual  or  germ  cells,  while  the  remaining  ones  divide 
among  themselves  the  various  functions  necessary  for  the  main- 
tenance of  the  individual,  and  may  be  termed  the  somatic  cells. 
The  germ  cells  represent  potentially  the  next  generation,  while 
the  somatic  cells  constitute  the  present  one.  The  idea  may  be 
represented  schematically  thus: 

First  generation 


Somatic  cells  -\-  germ  cells 

'  II 

Second  generation 

Soiiiatic  cells  -f-  germ  cells 

II 
Third  generation 


Somatic  cells  +  germ  cells,  etc. 


It  is  evident,  then,  while  the  somatic  cells  of  each  generation  die 
at  their  appointed  time  and  are  differentiated  anew  for  each  gene- 
ration from  the  germ  cells,  the  latter,  which  may  be  termed  collec- 
tively the  germ-plasm,  are  handed  on  from  generation  to  generation 
without  interruption,  and  it  may  be  supposed  that  this  has  been 


INTRODUCTION  9 

the  case  ab  initio.  This  is  the  doctrine  of  the  continuity  of  the 
germplasm,  a  doctrine  of  fundamental  importance  on  account  of  its 
bearings  on  the  phenomena  of  heredity. 

It  is  necessary,  however,  to  fix  upon  some  link  in  the  continuous 
chain  of  the  germ-plasm  as  the  starting-point  of  the  development 
of  each  individual,  and  this  link  is  the  fertilized  ovum.  By  this  is 
meant  a  germ  cell  produced  by  the  fusion  of  two  units  of  the  germ- 
plasm.  In  many  of  the  lower  forms  of  life  (e.g.  Hydra  and  certain 
turbellarian  worms)  reproduction  may  be  accomplished  by  a  di- 
vision of  the  entire  organism  into  two  parts  or  by  the  separation  of 
a  portion  of  the  body  from  the  parent  individual.  Such  a  method 
of  reproduction  is  termed  non-sexual.  Furthermore  in  a  number  of 
forms  {e.g.,  bees,  Phylloxera,  water-fleas)  the  germ  cells  are  able  to 
undergo  development  without  previously  being  fertilized,  this  con- 
stituting a  method  of  reproduction  known  as  parthenogenesis. 
But  in  all  these  cases  sexual  reproduction  also  occurs,  and  in  all  the 
more  highly  organized  animals  it  is  the  only  niethod  that  normally 
occurs;  in  it  a  germ  cell  develops  only  after  complete  fusion  with 
another  germ  cell.  In  the  simpler  forms  of  this  process  little 
difference  exists  between  the  two  combining  cells,  but  since  it  is, 
as  a  rule,  of  advantage  that  a  certain  amount  of  nutrition  should  be 
stored  up  in  the  germ  cells  for  the  support  of  the  developing  embryo 
until  it  is  able  to  secure  food  for  itself,  while  at  the  same  time  it  is 
also  advantageous  that  the  cells  which  unite  shall  come  from  differ- 
ent individuals  (cross-fertilization),  and  hence  that  the  cells 
should  retain  their  motility,  a  division  of  labor  has  resulted. 
Certain  germ  cells  store  up  more  or  less  food  yolk,  their  motility 
becoming  thereby  impaired,  and  form  what  are  termed  the  female 
cells  or  ova,  while  others  discard  all  pretensions  of  storing  up  nutri- 
tion, are  especially  motile  and  can  seek  and  penetrate  the  inert 
ova;  these  latter  cells  constitute  the  male  cells  or  spermatozoa.  In 
many  animals  both  kinds  of  cells  are  produced  by  the  same  indi- 
vidual, but  in  all  the  vertebrates  (with  rare  exceptions  in  some  of 
the  lower  orders)  each  individual  produces  only  ova  or  spermato- 
zoa, or,  as  it  is  generally  stated,  the  sexes  are  distinct. 

It  is  of  importance,  then,  that  the  peculiarities  of  the  two 


lO  INTRODUCTION 

forms  of  germ  cells,  as  they  occur  in  the  human  species,  should 
be  considered. 

LITERATURE 

R.  Chambers:  "Microdissection  Studies."  Amer.  Jour.  Physiol.,  xliii,  1917,  and 

Jour.  Exper.  ZooL,  xxiii,  191 7. 
E.  V.  Cowdry:  "The  general  functional  significance  of  mitochondria"  Amer.  Jour. 

Anat.,  xix,  1916. 
J.  Duesberg:  " Plastosomen,     Apparato     reticolare     interno,     und     Chromidial- 

apparat,"  Ergeb.  Anat.  u.  Emtw.,  xx,  191 1. 
J.  Duesberg:  "On  the  Present  Status  of  the  Chondriosome  Problem."     Biol. 

Bully  xxxvi,  1919. 
O.  Hertwig:  "Die  Zelle  und  die  Gewebe."     Jena,  1893. 
H.  L.  WiEMAN:  "Chromosomes  in  Man"  Amer.  Jour.  Anat.,  xiv,  19 13. 
E.    B.   Wilson:  "The    Cell   in    Development    and   Inheritance."     Third  edition 

New  York,  1900. 


PART  I 

GENERAL    DEVELOPMENT 


CHAPTER  I 


THE  SPERMATOZOON  AND  SPERMATOGENESIS;  THE 

OVUM  AND  ITS  MATURATION  AND 

FERTILIZATION 

The  Spermatozoon. — The  human  spermatozoon  (Figs.  4  and  5) 
is  a  minute  and  greatly  elongated  cell,  measuring  about  0.05  mm. 
in  length.  It  consists  of  an  anterior  broader  portion  or  head  (Fig. 
S,H),  which  measures  about  0.005  ^i^-  '^^  length  and,  when  viewed 
from  one  surface  (Fig.  4,  i),  has  an  oval  outline,  though  since  it  is 
somewhat  flattened  or  concave  toward  the  tip,  it  has  a  pyriform 
shape  when  seen  in  profile  (Fig.  4,  2).  Covering  the  flattened 
portion  of  the  head  and  fitting  closely  to  it  is  a  delicate  cap-like 
membrane,  the  head-cap  (Fig.  5,  H.C.),  whose  apex  is  a  sharp  edge, 
this  structure  corresponding  to  a  pointed  prolongation  of  the  cap 
found  in  the  spermatozoon  of  many  of  the  lower  vertebrates  and 
known  as  the  perforatorium.  Immediately  behind  the  head  is  a 
short  portion  known  as  the  neck  (Fig.  5,  iV),  which  consists  of  an 
upper  more  refractive  body,  the  anterior  nodule,  and  a  lower 
clearer  portion.  To  this  succeeds  the  connecting  or  middle-piece 
(Figs.  4,  m  and  5,  M)  which  begins  with  a  posterior  nodule,  from 
the  center  of  which  there  passes  back  through  the  axis  of  the  piece 
an  axial  filament,  enclosed  within  a  sheath,  this  latter  having 
wrapped  around  it  a  spiral  filament.  At  the  lower  end  of  the 
middle  piece  this  spiral  filament  terminates  in  the  annulus,  through 
which  the  axial  filament  and  its  sheath  passes  into  the  flagellum  or 
tail  (Fig.  4,  /).     This  portion,  which  constitutes  about  four-fifths 

II 


12 


THE    SPERMATOZOON 


of  the  total  length  of  the  spermatozoon  is  composed 'simply  of  the 
axial  filament  and  its  sheath,  this  latter  gradually  thinning  out  as 
it  passes  backward  and  ceasing  altogether  a  short  distance  above 
the  end  of  the  axial  filament.     The  filament  thus  projects  some- 


k 
\m 


Fig.  4. — Human  Spermatozoon. 
I,  Front  view;  2,  side  view  of 
the  head;  e,  terminal  filament; 
k,  head;  /,  tail;  m,  middle-piece. 
—{After  Retzius.) 


H.  { 


N. 


M. 


Fig.  5. — Diagram  Showing  tIie  Structure 
OF  A  Human  Spermatozoon. 
Af,  Axial  filament;  An,  annulus;  H,  head; 
H.  C,  lower  border  of  head-cap;  M,  middle - 
piece;  N,  neck;  Na  and  Np,  anterior  and  pos- 
terior nodule;  S,  sheath  of  axial  filament;  Spf, 
spiral  filament. — {Bonnet,  after  Meves.) 


what  beyond  the  actual  end  of  the  tail,  forming  what  is  known  as 
•  the  terminal  filament  or  end-piece  (Fig.  4,  e) . 

To  understand  the  significance  of  the  various  parts  entering 
into  the  composition  of  the  spermatozoon  a  study  of  their  develop- 
ment is  necessary,  and  since  the  various  processes  of  spermatogene- 
sis have  been  much  more  accurately  observed  in  such  mammalia  as 
the  rat  and  guinea-pig  than  in  man,  the  description  which  follows 
will  be  based  on  what  has  been  described  as  occurring  in  these 
forms.     From  what  is  known  of  the  spermatogenesis  in  man  it 


SPERMATOGENESIS 


13 


seems  certain  that  it  closely  resembles  that  of  these  mammals  so 
far  as  its  essential  features  are  concerned. 

Spermatogenesis. — ^The  spermatozoa  are  developed  from  the 
cells  which  line  the  interior  of  the  seminiferous  tubules  of  the  testis. 
The  various  stages  of  development  cannot  all  be  seen  at  any  one 
part  of  a  tubule,  but  the  formation  of  the  spermatozoa  seems  to 


sc^:^ 


sg- 


Fig.  6. — Diagram   Showing  Stages  gf  Spermatogenesis  as  Seen  in  Differ- 
ent Sections  of  a  Seminiferos  Tubule  of  a  Rat. 
sc^,   spermatocyte  of  the  first  order;  sc^,  spermatocyte  of  the  second  order; 


sg,  spermatogonium;  sp,  spermatid;  sz,  spermatozoon, 
pled. — (Adapted  from  Lenhossek.) 


The  Sertoli  cells  are  stip- 


pass  along  each  tubule  in  a  wave-like  manner  and  the  appearances 
presented  at  different  points  of  the  wave  may  be  represented  dia- 
grammatically  as  in  Fig.  6. 


14  SPERMATOGENESIS 

In  section  A  of  this  figure  four  different  generations  of  cells  are 
represented;  above  are  mature  spermatozoa  lying  in  the  lumen 
of  the  tubule,  while  next  the  basement  membrane  is  a  series 
of  cells  from  which  a  new  generation  of  spermatozoa  is  about  to 
develop.  The  cells  of  this  series  are  of  two  kinds;  the  stippled  one 
will  develop  into  a  structure  known  as  a  Sertoli  cell,  while  the  others , 
termed  spermatogonia  (sg) ,  are  the  parent  cells  of  both  spermatozoa 
and  Sertoli  cells.  The  spermatogonia  undergo  several  divisions 
before  becoming  the  actual  parent  cells  of  the  structures  men- 
tioned, and  it  is  found  that  about  one  in  four  of  the  ultimate  sper- 
matogonia contains  a  peculiar  rod-like  crystalloid,  the  crystalloid 
of  Lubarsch.  It  seems  probable  that  the  cells  possessing  these 
structures  are  the  parent  cells  of  the  Sertoli  cells.  For  the  latter 
also  contain  crystalloids,  these  being  of  two  kinds,  a  large  one,  the 
crystalloid  of  Charcot,  and  one  or  two  smaller  ones,  similar  to  the 
Lubarsch  crystalloids  of  the  parent  cells.  It  is  supposed  that 
both  kinds  of  crystalloids  have  been  formed  by  the  division  of  a 
Lubarsch  crystalloid  during  the  growth  of  the  Sertoli  cell.  This 
growth  is  very  rapid,  the  cells  increasing  greatly  in  size,  as  is  indi- 
cated in  Fig.  6,  and  branching  at  their  free  ends  to  ramify  around 
groups  of  sperm-cells,  each  Sertoli  cell  thus  coming  to  enclose  at 
its  free  end  twenty-four  spermatozoa,  to  which  it  acts  as  a  nurse, 
supplying  them  with  nutrition.  The  spermatozoa  when  mature 
are  set  free  in  the  lumen  of  the  seminiferous  tubule  and  their 
Sertoli  cell  then  degenerates.  Sertoli  cells,  therefore,  continue 
to  be  formed  throughout  the  period  of  sexual  activity,  new 
ones  for  each  generation  of  spermatozoa  being  formed  from  the 
spermatogonia. 

Those  ultimate  spermatogonia  that  do  not  contain  crystalloids 
are  the  parent  cells  of  the  spermatozoa  and  each  divides  into  two 
cells  termed  primary  spermatocytes,  indicated  in  sections  A  and 
B  of  Fig.  6  by  sc^.  In  the  section  C  these  cells  are  shown  dividing 
to  form  secondary  spermatocytes  (sc"^),  and  these  almost  imme- 
diately divide  again,  each  giving  rise  to  two  spermatids  {sp), 
which  later  become  directly  transformed  into  spermatozoa.  From 
each  primary  spermatocyte  there  are  formed,  therefore,  as  the 


SPERMATOGENESIS 


15 


result  of  two  mitoses,   four  cells,   each  of  which  represents  a 
spermatozoon. 

During  these  divisions  important  departures  from  the  typical 
method  of  mitosis  occur,  these  departures  leading  to  a  reduction  of 
the  chromosomes  in  each  spermatid  to  one-half  the  number 
occurring  in  the  somatic  cells.     The  general  plan  by  which  this  is 


Pig.  7. — Diagram  Illustrating  the  Reduction  of  the  Chromosomes   During 

Spermatogenesis. 
5c^  Spermatocyte  of  the  first  order;  sc^,  spermatocyte  of  the  second  order;  sp, 

spermatid. 

accomplished  may  be  described  as  follows:  In  the  division  of  the 
spermatogonia  the  number  of  chromosomes  that  appears  is  iden- 
tical with  that  found  in  the  somatic  cells,  so  that  in  a  form 
whose  somatic  number  is  eight,  eight  chromosomes  appear  in 
each  spermatogonium,  and  divide  so  that  eight  pass  to  each  of  the 
resulting  primary  spermatocytes.     When  these  cells  divide,  how- 


1 6  SPERMATOGENESIS 

ever,  the  number  of  chromosomes  that  appears  is  only  one-half 
the  somatic  number,  namely,  four  in  the  supposed  case  that  is 
being  described  (Fig.  7,  sc^).  The  further  history  of  these 
chromosomes  indicates  that  each  is  composed  of  four  elements 
more  or  less  closely  united  to  form  a  tetrad,  and  during  mitosis 
each  tetrad  divides  into  two  dyads,  four  of  which  will  therefore 
pass  into  each  secondary  spermatocyte. 

These  cells  (Fig.  7 ,  sc"^)  finally  undergo  a  division  in  which 
each  of  the  dyads  they  contain  is  halved,  so  that  each  sper- 
matid receives  a  number  of  single  chromosomes  equal  to  half  the 
number  characteristic  for  the  species  (Fig.  7,  sp). 

This  account  of  the  behavior  of  the  chromosomes  during 
spermatogenesis  assumes  that  all  the  chromosomes  of  the 
primary  spermatocytes  are  of  equal  value  and  behave  simil- 
arly during  mitosis.  It  has  been  found,  however,  that  in 
a  number  of  forms  (insects,  spiders,  birds,  mammals,  etc.) 
this  is  not  the  case  and  it  seems  probable  that  in  man 
also  certain  of  the  spermatocytic  chromosomes,  termed 
idiochromosomes,  differ  decidedly  from  their  fellows.  The 
exact  behavior  of  these  special  chromosomes  is  still  somewhat 
uncertain  so  far  as  the  human  species  is  concerned,  but  according 
to  the  recent  observations  of  Wieman  it  is  as  follows.  In  the 
spermatogonial  divisions  twenty-four  chromosomes  appear,  two 
of  which  are  idiosomes  and  for  convenience  may  be  denoted 
as  X  and  Y.  In  the  primary  spermatocyte  only  twelve  chromo- 
somes appear,  one  of  which  is  the  now  paired  XY  element,  and 
in  the  metaphase  this  element  divides  longitudinally,  one-half 
passing  to  each  pole  of  the  mitotic  spindle  (Fig.  8,  XY^  and  XY^) . 
In  the  secondary  spermatocyte  twelve  chromosomes  again  appear 
and  in  the  metaphase  all  divide,  the  X  idiochromosome  separating 
from  the  Y  and  passing  to  the  opposite  spindle  pole.  Each  sper- 
matid thus  contains  eleven  ordinary  chromosomes,  but  in  addi- 
tion half  of  them  contain  an  X  idiochromosome,  while  the  other 
half  contain  a  Y  idiochromosome  (Fig.  8). 

Guyer  and  Montgomery  have,  however,  obtained  quite  differ- 
ent results.     According  to  the  former  the  two  idiochromosomes  do 


SPERMATOGENESIS 


17 


not  divide  in  the  primary  spermatocyte  division,  but  pass  un- 
changed to  one  pole  of  the  spindle,  so  that  half  the  secondary 
spermatocytes  contain  idiochromosomes  and  the  other  half  do 
not.  Since  all  the  chromosomes  of  the  secondary  spermatocyte 
divide  there  will  thus  be  two  classes  of  spermatids,  one  possessing 
ten  ordinary  chromosomes  and  the  other  possessing  these  plus 


Fig.  8. — Diagram  Illustrating  Human  Spermatogenesis. 

The  upper  figure  represents  a  metaphase  in  the  primary  spermatocyte  it  which 
1 1  ordinary  chromosomes  from  the  equatorial  plate,  the  xy  element  having  already 
divided  the  daughter  elements  passing  to  opposite  poles.  The  middle  figures  repre- 
sent the  equatorial  plates  of  two  secondary  spermatocytes  each  consisting  of  11 
ordinary  chromosomes  and  an  xy  element.  TJie  lower  figures  show  the  chromo- 
some constituents  of  the  spermatids,  each  comaining  n  ordinary  chromosomes, 
but  half  of  them  with  an  x  element  and  a  half  with  a  y  element. — {Based  on 
Wietnan.) 

two  idiochromosomes.  Montgomery  on  the  other  hand  believed 
that  there  was  a  good  deal  of  variety  in  the  behavior  of  the 
idiochromosomes  and  held  that  there  were  at  least  four,  and  possi- 


l8  SPERMATOGENESIS 

bly  five  or  six,  classes  of  spermatids,  and  the  matter  is  further 
complicated  by  the  results  obtained  by  von  Winiwarter,  according 
to  which  the  primary  spermatocytes  possessed  twenty-four  chro- 
mosomes instead  of  twelve.  One  of  these  twenty-four  was  an 
idiochromosome,  which  on  division  passed  undivided  into  one  of 
the  secondary  spermatocytes,  so  that  of  these  cells  there  were  two 
classes,  one  containing  twenty-four  chromosomes  and  the  other 
twenty-three,  and  this  condition  was  transmitted  to  the  sper- 
matids. These  very  divergent  results  are  greatly  in  need  of 
thorough  revision,  but  one  feature  is  common  to  all  in  that  two 
classes  of  spermatids  are  recognized,  differing  in  their  chromosomal 
constituents.     The  significance  of  this  will  be  considered  later 

(P-  32). 

The  transformation  of  the  spermatids  into  spermatozoa  takes 
place  while  they  are  in  intimate  association  with  the  Sertoli  cells, 
a  number  of  them  fusing  with  the  cytoplasm  of  an  enlarged  Sertoli 
cell,  as  shown  in  Fig.  6,  s,  and  probably  receiving  nutrition  from  it. 
In  each  spermatid  there  is  present  in  addition  to  the  nucleus,  an 
archoplasm  sphere,  two  centrosomes  that  have  migrated  from 
the  archoplasm  and  lie  free  in  the  cytoplasm,  and  numerous 
chondriosomes.  The  centrosomes  and  the  archoplasm  sphere 
take  up  their  position  at  opposite  poles  of  the  nucleus,  the  archo- 
plasm eventually  forming  the  head-cap  of  the  spermatozoon,  and 
from  one  of  the  centrosomes  a  slender  axial  filament  grows  out  and 
soon  projects  beyond  the  limits  of  the  cytoplasm  (Fig.  g,  A). 
The  other  centrosome  becomes  a  rod-shaped  structure  which  ap- 
plies itself  closely  to  the  posterior  pole  of  the  nucleus,  becoming  the 
anterior  nodule,  while  the  lower  one,  from  which  the  filament  arises 
becomes  at  first  pyramidal  in  shape  (Fig.  9,  B)  and  later  separates 
into  a  rod-like  portion  to  which  the  filament  is  attached  and  a  ring, 
through  which  the  filameAt  passes  (Fig.  9,  C).  The  rod-like 
portion  becomes  the  posterior  nodule,  and  the  ring  separates  from 
it  to  form  the  annulus  (Fig.  9,  D).  The  nucleus  becomes  the 
head  of^the  spermatozoon,  the  cytoplasm  surrounding  it  becoming 
reduced  to  an  exceedingly  delicate  layer,  so  that  the  head  is  com- 
posed almost  entirely  of  nuclear  substance,  if  the  head-cap  be  left 


THE    OVUM 


19 


out  of  consideration.  The  spiral  filament  of  the  middle-piece  is, 
however,  formed  from  the  cytoplasmic  chondrosomes,  and 
according  to  some  authors  these  also  furnish  the  material  for  the 
sheath  of  the  axial  filament,  though  this  has  been  denied  (Meves)7 
the  sheath  being  regarded  a  differentiation  of  the  axial  filament. 
Each  spermatozoon  is,  then,  one  of  four  equivalent  cells,  produced 
by  two  successive  divisions  of  a  primary  spermatocyte  and  con- 
taining approximately  one-half  the  number  of  chromosomes 
characteristic  for  the  species. 


Fig.  9. 


-Stages  in  the  Transformation  of  a  Spermatid  into  a  Spermatozoon. — 

(After  Meves.) 


The  number  of  spermatozoa  produced  during  the  lifetime  of  a 
single  individual  is  very  large.  It  has  been  found  that  i  cu. 
mm.  of  human  ejaculate  contains  60,876  spermatozoa,  a  single 
ejaculate,  therefore,  containing  over  200,000,000.  This  would 
indicate  that  during  his  lifetime  a  man  may  produce  340  billion 
spermatozoa  (Lode). 

The  Ovum. — The  human  ovum  is  a  spherical  cell  measuring 
about  0.2  mm.  in  diameter  and  is  contained  within  a  cavity  situ- 
ated near  or  at  the  surface  of  the  ovary  and  termed  a  Graafian 
follicle.  This  follicle  is  surrounded  by  a  capsule  composed  of  two 
layers,  an  outer  one,  the  theca  externa,  consisting  of  fibrous  tissue 
resembling  that  found  in  the  ovarian  stroma,  and  an  inner  one,  the 
theca  interna,  composed  of  numerous  spherical  and  fusiform  cells. 


20 


THE    OVUM 


Both  the  thecae  are  richly  supplied  with  blood-vessels,  the  theca 
interna  especially  being  the  seat  of  a  very  rich  capillary  network. 
Internal  to  the  theca  interna  there  is  a  transparent,  thin,  and 
structureless  hyaline  membrane,  within  which  is  the  follicle  proper, 
whose  wall  is  formed  by  a  layer  of  cells  termed  the  stratum  granu- 
losum  (Fig.  lo,  mg)  which  inclose  a  cavity  filled  with  an  albuminous 
fluid,  the  liquor  folliculi.     At  one  point,  usually  on  the  surface 


Pig.  10. — Section  through  Portion  of  an  Ovary  of  an  Opossum  (Didephys  vir- 

giniana)  showing  Ova  and  Follicles  in  Various  Stages  of  Development. 
b.  Blood-vessel;  dp,  discus  proligerus;  mg,  stratum  granulosum;  o,  ovum;  s,  stroma; 

th,  theca  folliculi. 


nearest  the  center  of  the  ovary,  the  stratum  granulosum  is  greatly 
thickened  to  form  a  mass  of  cells,  the  discus  proligerus  (dp),  which 
projects  into  the  cavity  of  the  follicle  and  encloses  the  ovum  (a). 
Usually  but  a  single  ovum  is  contained  in  any  discus,  though 
occasionally  two  or  even  three  may  occur. 

The  cells  of  the  discus  proligerus  are  for  the  most  part  more  or 
less  spherical  or  ovoid  in  shape  and  are  arranged  irregularly.     In 


THE    OVUM 


21 


the  immediate  vicinity  of  the  ovum,  however,  they  are  more  co- 
lumnar in  form  and  are  arranged  in  about  two  concentric  rows,  thus 
giving  a  somewhat  radiated  appearance  to  this  portion  of  the  dis-_ 
cus,  which  is  termed  the  corona  radiata  (Fig.  1 1,  cr).  Immediately 
within  the  corona  is  a  transparent  membrane,  the  Zona  pellucida 
(Fig.  II,  Zp),  about  as  thick  as  one  of  the  cell  rows  of  the  corona 
(0.02  to  0.024  mm.),  and  presenting  a  very  fine  radial  striation 

zp 


Fig.  II. — Ovum  from  Ovary  of  a  Woman  Thirty  Years  of  Age. 

cr.  Corona  radiata;  n,  nucleus;  p,  protoplasmic  zone  of  ovum;  ps,  perivitelline  space; 

y,  yolk;  zp,  zona  pellucida. —  (Nagel.) 

which  has  been  held  to  be  due  to  minute  pores  traversing  the  mem- 
brane and  containing  delicate  prolongations  of  the  cells  of  the 
corona  radiata.  Within  the  zona  pellucida  is  the  ovum  proper, 
whose  cytoplasm  is  more  or  less  clearly  differentiated  into  an  outer 
more  purely  protoplasmic  portion  (Fig.  11,  p)  and  an  inner  mass 
(y)  which  contains  numerous  fine  granules  of  fatty  and  albuminous 
natures.     These  granules  represent  the  food  yolk  or  deutoplasm, 


2  2  OVULATION   AND    THE    CORPUS    LUTEUM 

which  is  usually  much  more  abundant  in  the  ova  of  other  mammals 
and  forms  a  mass  of  relatively  enormous  size  in  the  ova  of  birds 
and  reptiles.  The  nucleus  (n)  is  situated  somewhat  excentrically 
in  the  deutoplasmic  portion  of  the  ovum  and  contains  a  single, 
well-defined  nucleolus. 

A  folHcle  with  the  structure  described  above  and  containing  a 
fully  grown  ovum  may  measure  anywhere  from  five  to  twelve 
millimeters  in  diameter,  and  is  said  to  be  ^'mature/'having  reached 
its  full  development  and  being  ready  to  burst  and  set  free  the 
ovum.  This,  however,  is  not  yet  mature;  it  is  not  ready  for  fer- 
tilization, but  must  first  undergo  certain  changes  similar  to  those 
through  which  the  spermatocyte  passes,  the  so-called  ovum  at 
this  stage  being  more  properly  a  primary  oocyte.  But  before 
describing  the  phenorrena  of  maturation  of  the  ovum  it  will  be 
well  to  consider  the  extrusion  of  the  ovum  and  changes  which 
the  follicle  subsequently  undergoes. 

Ovulation  and  the  Corpus  Luteum. — As  a  rule,  but  a  single 
follicle  near  maturity  is  found  in  either  the  one  or  the  other  ovary 

at  any  given  time.  In  the 
early  stages  of  its  develop- 
ment a  follicle  is  situated 
somewhat  deeply  in  the  stroma 
of  the  ovary,  but  as  the  liquor 
folliculi  increases  in  amount  a 
tension  is  produced  within  the 
follicle  which  causes  it  to  en- 
large especially  in  the  direction 
of  least  resistance,  that  is  to- 
pic  i2.-0vary  OF  A  Woman  Nine-   ^^rds  the  surface  of  the  ovary, 

TEEN  Years  of  Age,  Eight  Days  after  -^ 

Menstruation.  where   it   eventually   forms    a 

d.  Blood-clot;  /,  Graafian  follicle;  th,       marked  prominence  and  its  con- 

theca. — {Kollmann.) 

tents  are  separated  from  the  ab- 
dominal cavity  only  by  an  exceedingly  thin  membrane.  This 
membrane  finally  ruptures,  and  the  liquor  folliculi  rushes  out 
through  the  rupture,  carrying  with  it  the  ovum  surrounded  by 
some  of  the  cells  of  the  discus  prohgerus. 


OVULATION  AND  THE  CORPUS  LUTEUM  25 

The  immediate  cause  of  the  bursting  of  the  follicle  has  usually 
been  ascribed  to  the  tension  within  the  follicle,  due  to  the  increase 
of  the  liquor  folliculi,  finally  reaching  the  bursting  point.  It 
has  been  shown  that  the  liquor  folliculi  of  the  pig  has  a  distinct 
digestive  action  on  ovarial  tissue  (Schochet)  and  this  would 
play  an  important  part  in  both  the  growth  and  rupture  of  the 
follicle,  and,  furthermore,  it  must  not  be  forgotten  that  the  ovarial 
stroma  contains  a  considerable  quantity  of  non-striped  muscle 
tissue,  a  spasmodic  contraction  of  which  would  produce  a  sudden 
increase  of  the  intra-follicular  tension. 

Normally  the  ovum  when  expelled  from  its  follicle  is  received 
at  once  into  the  Fallopian  tube,  and  so  makes  its  way  to  the  uterus, 
in  whose  cavity  it  undergoes  its  development.  Occasionally,  how- 
ever, this  normal  course  may  be  interfered  with,  the  ovum  coming 
to  rest  in  the  tube  and  there  undergoing  its  development  and 
producing  a  tubal  pregnancy;  or,  again,  the  ovum  may  not  find 
its  way  into  the  Fallopian  tube,  but  may  fall  from  the  follicle  into 
the  abdominal  cavity,  where,  if  it  has  been  fertilized,  it  will  under- 
go development,  producing  an  abdominal  pregnancy;  and,  finally, 
and  still  more  rarely,  the  ovum  may  not  be  expelled  when  the 
Graafian  follicle  ruptures  and  yet  may  be  fertilized  and  undergo 
its  development  within  the  follicle,  bringing  about  what  is  termed 
an  ovarian  pregnancy.  All  these  varieties  of  extra-uterine  preg- 
nancy are,  of  course,  exceedingly  serious,  since  in  none  of  them  is 
the  fetus  viable. 

With  the  setting  free  of  the  ovum  the  usefulness  of  the  Graafian 
follicle  is  at  an  end,  and  it  begins  at  once  to  undergo  retrogressive 
changes  which  result  primarily  in  the  formation  of  a  structure 
known  as  the  corpus  luteum  (Fig.  12).  On  the  rupture  of  the 
follicle  a  considerable  portion  of  the  stratum  granulosum  remains 
in  place,  and  the  cells  composing  it  undergo  proliferation  and 
develop  in  their  substance  a  yellow  pigment  known  as  lutein,  the 
color  imparted  to  the  follicle  by  this  substance  having  suggested 
the  name,  corpus  luteum,  that  is  now  applied  to  it.  The  blood- 
vessels of  the  theca  interna  become  enlarged  and  hernia-like  pro- 
trusions from  them  penetrate  between  the  proliferating  granulosa 


24 


OVULATION  AND  THE  CORPUS  LUTEUM 


cells,  carrying  with  them  a  certain  amount  of  connective  tissue. 
Extravasations  from  the  capillaries  into  the  cavity  of  the  follicle 
take  place  during  the  early  stages  of  the  vascularization  of  the 
granulosa,  but  in  time  the  entire  cavity  of  the  follicle  becomes  filled 
with  lutein  cells,  separated  into  groups  by  trabeculae  of  connective 
tissue  containing  blood-vessels,  the  corpus  luteum  thus  reaching 
its  maturity  (Fig.  13). 


Fig.  13.— Section  through  the  Corpus  Luteum  of  a  Rabbit,  Seventy  Hours 

post  coitum. 
The  cavity  of  the  follicle  is  almost  completely  filled  with  lutein  cells  among  which 
is  a  certain  amount  of  connective  tissue,     g,  Blood-vessels;  ke,  ovarial  epithelium. — 
{Sohotta.) 


In  later  stages  there  is  a  gradual  increase  in  the  amount  of  con- 
nective tissue  present  and  a  corresponding  diminution  of  the  lutein 
cells,  the  corpus  luteum  gradually  losing  its  yellow  color  and  be- 
coming converted  into  a  whitish,  fibrous,  scar-like  body,  the  corpus 
albicans,   which   may   eventually  almost   completely   disappear. 


OVULATION  AND  THE  CORPUS  LUTEUM  2$ 

These  various  changes  occur  m  every  ruptured  follicle,  whether  or 
not  the  ovum  which  was  contained  in  it  be  fertilized.  But  the 
rapidity  with  which  the  various  stages  of  retrogression  ensue  differs 
greatly  according  to  whether  pregnancy  occurs  or  not,  and  it  is 
customary  to  distinguish  the  corpora  lutea  which  are  associated 
with  pregnancy  as  corpora  lutea  vera  from  those  whose  ova  fail  to 
be  fertilized  and  which  form  corpora  lutea  spuria.  In  the  latter 
the  retrogression  of  the  follicle  is  completed  usually  in  about  five 
or  six  weeks,  while  the  corpora  vera  persist  throughout  the  entire 
duration  of  the  pregnancy  and  complete  their  retrogression  after 
the  birth  of  the  child. 

In  the  account  of  the  development  of  the  corpus  luteum  given 
above  the  granulosa  cells  are  described  as  being  converted  into  the 
lutein  cells.  This  is  the  opinion  originally  advanced  by  Bischoff, 
but  another,  which  was  held  by  von  Baer,  was  for  a  time  m.ore 
generally  accepted.  It  maintained  that  the  granulosa  cells  quickly 
underwent  degeneration,  the  lutein  cells  and  the  entire  mass  of  the 
corpus  luteum  being  formed  from  the  theca  interna.  The  thor- 
ough study  of  the  phenomena  by  Sobotta  (1897)  in  a  perfect 
series  of  mouse  ovaries  demonstrated  that  in  that  form  the  granu- 
losa cells  persist  and  become  converted  into  lutein  cells,  and  later 
observations  on  other  mammals,  such  as  the  rabbit  (Sobotta), 
certain  bats  (Van  der  Stricht),  the  sheep  (Marshall) ,  Spermophile 
(Volker,  Drips),  guinea-pig  (Sobotta,  L.  Loeb)  and  various  mar- 
supials (Sandes,  O'Donaghue)  confirmed  the  correctness  of 
Sobotta*s  conclusions.  Adverse  results  were  obtained  from  the 
study  of  the  human  corpus  luteum  by  Clarke  and  from  that 
of  the  pig  by  Jankowski,  but  the  more  recent  observations  of  R . 
Mayer  (191 1)  make  it  altogether  probable  that  in  man,  also,  the 
granulosa  are  the  chief  source  of  the  lutein  cells  and^Corner's 
(19 1 5)  results  lead  him  to  believe  that  in  the  pig  the  granulosa 
cells  persist  and  contribute  to  the  formation  of  lutein.  The 
participation  of  theca  cells  in  the  lutein  formation  is  not,  however, 
excluded,  the  hard  and  fast  distinction  frequently  made  between 
granulosa  and  thecal  cells  being  probably  unwarranted. 

The  persistance  of  the  corpus  luteum  throughout  the  period 


26  THE   RELATION    OF    OVULATION   TO   MENSTRUATION 

of  pregnancy  and  its  disappearance  within  a  few  weeks  if  preg- 
nancy failed  to  supervene,  have  suggested  the  probability  of  its 
being  an  organ  of  internal  secretion  directly  concerned  in  the  pro- 
duction of  certain  of  the  changes  associated  with  pregnancy.  It 
has  been  found  that  experimental  removal  of  the  corpus  luteum  in 
rabbits  either  before  or  shortly  after  the  implantation  of  the  ovum 
in  the  wall  of  the  uterus  produces  a  failure  of  pregnancy  (Fraenkel) 
and  similar  results  have  been  obtained  in  mice  and  bitches  (Mar- 
shall and  Jolly)  and  in  Spermophiles  (Drips).  The  cessation  of 
ovulation  which  is  characteristic  of  pregnancy  has  also  been  as- 
cribed to  the  action  of  the  corpora  lutea  and  there  is  experimental 
evidence  in  support  of  such  a  view  (L.  Loeb).  But  while  the 
available  evidence  points  to  the  existence  of  an  internal  secretion 
by  the  corpora  lutea  and  to  its  having  some  influence  in  deter- 
mining the  conditions  associated  with  a  successful  pregnancy,  the 
precise  nature  of  its  action  is  still  obscure. 

The  Relation  of  Ovulation  to  Menstruation. — It  has  long 
been  believed  that  ovulation  is  coincident  with  certain  periodic 
changes  of  the  uterus  which  constitute  what  is  termed  menstrua- 
tion. This  phenomenon  makes  its  appearance  at  the  time  of 
puberty,  the  exact  age  at  which  it  appears  being  determined  by 
individual  and  racial  peculiarities  and  by  climate  and  other  fac- 
tors, and  after  it  has  once  appeared  it  normally  recurs  at  definite 
intervals  more  or  less  closely  corresponding  with  lunar  months  {i.e., 
at  intervals  of  about  twenty-eight  days)  until  somewhere  in  the 
neighborhood  of  the  fortieth  or  forty-fifth  year,  when  it  ceases. 

In  each  menstrual  cycle  four  stages  may  be  recognized,  one  of 
which,  the  intermenstrual,  greatly  exceeds  the  others  in  its  duration, 
occupying  about  one-half  the  entire  period.  During  this  stage  the 
mucous  membrane  of  the  uterus  is  practically  at  rest,  but  toward 
its  close  the  membrane  gradually  begins  to  thicken  and  the  second 
stage,  the  premenstrual  stage,  then  supervenes.  This  lasts  for  six 
or  seven  days  and  is  characterized  by  a  marked  proliferation  and 
swelling  of  the  uterine  mucosa,  the  subjacent  tissue  becoming  at 
the  same  time  highly  vascular  and  eventually  congested.  The 
walls  of  the  blood-vessels  situated  beneath  the  mucosa  then  degen- 


THE    RELATION    OF    OVULATION   TO    MENSTRUATION  27 

erate  and  permit  the  escape  of  blood  here  and  there  beneath  the 
mucous  membrane,  this  leading  to  the  third,  or  menstrual,  stage  in 
which  the  mucous  membrane  diminishes  in  thickness,  those  por- 
tions of  it  that  overlie  the  effused  blood  undergoing  fatty  degenera- 
tion and  desquamation,  so  that  the  stage  is  characterized  by  more 
or  less  extensive  hemorrhage.  The  duration  of  this  stage  is  from 
three  to  five  days  and  then  ensues  the  postmenstrual  stage,  lasting 
from  four  to  six  days,  during  which  the  mucous  membrane  is  re- 
generated and  again  returns  to  the  intermenstrual  condition. 

It  seems  but  natural  to  regard  these  changes  as  the  expression 
of  a  periodic  attempt  to  prepare  the  uterus  for  the  reception  of  the 
fertilized  ovum,  this  preparation  being  completed  during  the  pre- 
menstrual stage,  the  succeeding  menstrual  and  postmenstrual 
phenomenon  being  merely  the  return  of  the  uterine  mucosa  to  the 
resting  intermenstrual  stage,  pregnancy  not  having  occurred.  If 
this  be  the  real  significance  of  the  menstrual  cycle,  one  would  ex- 
pect to  find  ovulation  occurring  at  a  more  or  less  definite  portion 
of  the  cycle,  at  such  a  time  that  the  ovum,  if  fertilized,  would  be 
able  to  make  use  of  the  premenstrual  preparation  for  its  reception. 

Since  the  occurence  of  a  corpus  luteum  is  the  result  of  an  ovula- 
tion and  the  age  of  the  former  can  be  determined  within  certain 
limits  from  its  histological  appearance,  a  comparison  of  the  age  of 
the  corpus  luteum  with  the  condition  of  the  uterine  mucosa  should 
indicate  the  period  in  the  menstrual  cycle  when  ovulation  occurred . 
In  other  words  since  the  development  of  the  corpus  luteum  and  the 
menstrual  modifications  of  the  uterine  mucous  membrane  are 
both  cylical  phenomena,  the  question  arises  as  to  whether  any 
correlation  exists  between  the  two  and  therefore  between  the  proc- 
ess of  ovulation  and  the  condition  of  the  uterine  mucosa.  It  has 
been  a  very  general  belief  that  in  the  human  species  ovulation  as 
a  rule  occurred  at  about  the  time  of  the  menstrual  flow,  that  is  to 
say,  just  before,  during,  or  just  after  the  third  stage  of  the  men- 
strual cycle.  Fraenkel,  however,  studied  the  condition  of  the 
corpus  luteum  in  eighty-five  cases  in  which  the  ovaries  were  re- 
moved in  the  course  of  operations  and  found  that  in  ten,  in 
which  the  operation  had  been  performed  immediately  before  or 


28  THE    MATURATION    OF    THE    OVUM 

after  menstruation,  no  corpus  luteum  was  present  and  that  in 
twenty  in  which  a  newly  formed  corpus  luteum  was  found,  the  last 
menstruation  had  occured  on  the  average  nineteen  (13-27)  days 
previously.  The  criteria  adopted  by  Fraenkel  for  determining 
the  age  of  the  corpora  lutea  do  not  seem  to  have  been  sufficiently 
precise  and  later  observers  confirming  his  conclusion  that  ovula- 
tion corresponded  with  a  definite  stage  of  the  menstrual  cycle, 
referred  its  occurrence  to  a  somewhat  earlier  period  of  the  cycle. 
Thus  Willemin  concluded  that  it  occurred  at  about  fifteen  days 
after  the  beginning  of  menstruation;  Grosser,  not  later  than  six- 
teen days;  C.  Ruge  II,  within  fourteen  days;  and  Schroeder 
between  fourteen  and  sixteen  days. 

These  numbers  represent,  of  course,  an  average  from  which 
there  may  be  considerable  deviation  on  either  side,  but  they 
indicate  that  ovulation  in  the  human  species  corresponds  on  the 
average  with  the  middle  of  the  intermenstrual  period.  The  corpus 
luteum,  accordingly,  reaches  its  mature  development  during  the 
premenstrual  stage  and  may,  therefore,  be  the  determining  cause 
of  that  stage  (Schroeder) . 

In  lower  animals  ovulation  is,  as  a  rule  associated  with  a  certain 
condition  known  as  cesirus  or  *'heat,"  this  being  preceded  by  phenomena 
constituting  what  is  termed  procestrum  and  corresponding  essentially 
to  menstruation.  In  several  forms,  such  as  the  dog,  pig,  horse  and 
cow,  ovulation  occurs  regularly  in  association  with  "heat,"  but  in 
others,  such  as  the  cat,  the  ferret  and  the  rabbit,  it  occurs  at  this  time 
only  if  copulation  also  occurs.  In  the  case  of  ;monkeys,  although  the 
females  menstruate  regularly  throughout  the  year  there  is  nevertheless 
but  one  annual  cestral  period  when  ovulation  take  place  (Heape). 

The  Maturation  of  the  Ovimi.— Returning  now  to  the  ovum, 
it  has  been  shown  that  at  the  time  of  its  extrusion  from  the  Graa- 
fian follicle  it  is  not  equivalent  to  a  spermatozoon  but  to  a  primary 
spermatocyte,  and  it  may  be  remembered  that  such  a  spermatocyte' 
becomes  converted  into  a  spermatozoon  only  after  it  has  under- 
gone two  divisions,  during  which  there  is  a  reduction  of  the 
number  of  the  chromosomes  to  practically  one-half  the  number 
characteristic  for  the  species. 


THE    MATURATION    OF   THE    OVUM  29 

Similar  divisions  and  a  similar  reduction  of  the  chromosomes 
occur  in  the  case  of  the  ovum,  constituting  what  is  termed  its 
maturation.  The  phenomena  have  not  as  yet  been  observed  in 
human  ova,  and,  indeed,  among  mammals  only  with  any  approach 
to  completeness  in  comparatively  few  forms  (rat,  mouse,  guinea- 
pig,  bat  and  cat) ;  but  they  have  been  observed  in  so  many  other 
forms,  both  vertebrate  and  invertebrate,  and  present  in  all  cases 


Fig.  14. — Ovum  of  a  Mouse  Showing  the  Maturation  Spindle. 

The    ovum  is  enclosed  by  the  zona  pellucida  (z.  p),  to  which    the  cells  of    the 

corona   radiata  are  still  attached. — {Sohotta.) 

SO  much  uniformity  in  their  general  features,  that  there  can  be 
little  question  as  to  their  occurrence  in  the  human  ovum. 

In  typical  cases  the  ovum  (the  primary  oocyte)  undergoes  a 
division  in  the  prophases  of  which  the  chromatin  aggregates  to 
form  half  as  many  tetrads  as  there  are  chromosomes  in  the  somatic 
cells  (Fig.  15,  oc^)  and  at  the  metaphase  a  dyad  from  each  tetrad 
passes  into  each  of  the  two  cells  that  are  formed.     These  two  cells 


30 


THE    MATURATION    OF    THE    OVUM 


(secondary  oocytes)  are  not,  however,  of  the  same  size;  one  of 
them  is  almost  as  larg€  as  the  original  primary  oocyte  and  con- 
tinues to  be  called  an  ovum  {oc'^),  while  the  other  is  very  small  and 
is  termed  3i  polar  globule  (p).  A  second  division  of  the  ovum 
quickly  succeeds  the  first  (Fig.  15,  oc"^),  and  each  dyad  gives  a 


Fig.   15. — Diagram  Illustrating  the  Reduction  of  the  Chromosomes  during 

THE  Maturation  of  the  Ovum. 
0,  Ovum;  ocS  oocyte  of  the  first  generation;  oc^,  oocyte  of  the  second   generation; 

p,  polar  globule. 

single  chromosome  to  each  of  the  two  cells  which  result,  so  that 
each  of  these  cells  possess  half  the  number  of  chromosomes  charac- 
teristic for  the  species.  The  second  division,  like  the  first,  is 
unequal,  one  of  the  cells  being  relatively  very  large  and  constituting 
the  mature  ovum,  while  the  other  is  small  and  is  the  second  polar 


THE    FERTILIZATION    OF    THE    OVUM  3 1 

globule.  Frequently  the  first  polar  globule  divides  during  the 
formation  of  the  second  one,  a  reduction  of  its  dyads  to  single 
chromosomes  taking  place,  so  that  as  the  final  result  of  the  matura- 
tion four  cells  are  formed  (Fig.  15),  the  mature  ovum  {0),  and 
three  polar  globules  {p),  each  of  which  contains  half  the  number 
of  chromosomes  characteristic  for  the  species. 

The  similarity  of  the  maturation  phenomena  to  those  of  sper- 
matogenesis may  be  perceived  from  the  following  diagram: 

nX"^  Spermalo- 

I       1  cyte  I 


Oocyte  II         ()  O  ()  C)         ^Pfyuir 


OvuniO        O  00  00  O     O  Spermatids 

Polar  globules 

In  both  processes  the  number  of  cells  produced  is  the  same  and  in 
both  there  is  a  similar  reduction  of  the  chromosomes.  But  while 
each  of  the  four  spermatids  is  functional,  the  three  polar  globules 
are  non-functional,  and  are  to  be  regarded  as  abortive  ova,  formed 
during  the  process  of  reduction  of  the  chromosomes  only  to  under- 
go degeneration.  In  other  words,  three  out  of  every  four  potential 
ova  sacrifice  themselves  in  order  that  the  fourth  may  have  the 
bulk,  that  is  to  say,  the  amount  of  nutritive  material  and  cyto- 
plasm necessary  for  efficient  development. 

The  Fertilization  of  the  Ovum. — It  is  perfectly  clear  that  the 
reduction  of  the  chromosomes  in  the  germ  cells  cannot  very  long 
be  repeated  in  successive  generations  unless  a  restoration  of  the 
original  number  takes  place  occasionally,  and,  as  a  matter  of  fact, 
such  a  restoration  occurs  at  the  very  beginning  of  the  develop- 
ment of  each  individual,  being  brought  about  by  the  union  of  a 
spermatozoon  with  an  ovum.  This  union  constitutes  what  is 
known  as  the  fertilization  of  the  ovum. 


32  THE    FERTILIZATION    OF    THE    OVUM 

The  fertilization  of  the  human  ovum  has  not  been  observed, 
but  the  phenomenon  has  been  repeatedly  studied  in  lower  forms, 
and  thorough  studies  of  the  process  have  been  made  on  the  mouse 
and  the  guinea-pig.  The  results  obtained  from  these  are  taken 
as  a  basis  for  the  following  account. 

The  maturation  of  the  ovum  is  quite  independent  of  fertiliza- 
tion, but  in  many  forms  the  penetration  of  the  spermatozoon  into 
the  ovum  takes  place  before  the  maturation  phenomena  are  com- 
pleted. This  is  the  case  with  the  mouse.  A  spermatozoon  makes 
its  way  through  the  zona  pellucida  and  becomes  embedded  in  the 
cytoplasm  of  the  .ovum  and  its  tail  is  quickly  absorbed  by  the  cyto- 
plasm while  its  nucleus  and  probably  the  middle-piece  persist  as 
distinct  structures.  As  soon  as  the  maturation  divisions  are 
completed  the  nucleus  of  the  ovum,  now  termed  the  female  pro- 
nucleus (Fig.  1 6,  ek)^  migrates  toward  the  center  of  the  ovum,  and 
is  now  destitute  of  an  archoplasm  sphere  and  centrosome,  these 
structures  having  disappeared  after  the  completion  of  the  matura- 
tion divisions.  The  spermatozoon  nucleus,  which,  after  it  has 
penetrated  the  ovum,  is  termed  male  pronucleus  (spk),  may  lie  at 
first  at  almost  any  point  in  the  peripheral  part  of  the  cytoplasm, 
and  it  now  begins  to  approach  the  female  pronucleus,  preceded 
by  the  middle-piece,  which  becomes  an  archoplasm  sphere  with 
its  contained  centrosome  and  is  surrounded  by  astral  rays.  The 
two  pronuclei  finally  come  into  contact  near  the  center  of  the 
ovum,  forming  what  is  termed  the  segmentation  nucleus  (Fig.  i6), 
the  archoplasm  sphere  and  centrosome  which  have  been  introduced 
with  the  spermatozoon  undergo  division,  the  two  archoplasm 
spheres  so  formed  migrate  to  opposite  poles  of  the  segmentation 
nucleus,  an  amphiaster  forms  and  the  compound  nucleus  passes 
through  the  various  prophases  of  mitosis. 

In  describing  the  spermatogenesis  it  was  shown  (p.  i6)  that 
two  classes  of  spermatozoa  were  formed  in  man,  those  of  one  class 
containing  an  X  idiochromosome  and  the  other  a  Y  element,  or 
if  Guyer's  results  should  prove  to  be  more  correct,  one  class  con- 
taining ten  ordinary  chromosomes  plus  two  idiochromosomes, 
while  the  other  possesses  only  ordinary  chromosomes.     A  similar 


THE    FERTILIZATION    OF   THE    OVUM 


33 


Fig.  i6. — Six  Stages  in  the  Process  of  Fertilization  of  the  Ovum  of  a  Mouse. 
After  the  first  stage  figured  it  is  impossible  to  determine  which  of  the  two  nuclei 
represents  the  male  or  female  pronucleus,     ek.  Female  pronucleus;  rki  and  rki,  polar 
globules;  spk,  male  pronucleus. — {Sobotta.) 


34 


tHE    FERTILIZATION    OF    THE    OVUM 


separation  of  the  ova  into  two  classes  probably  does^not  occur,  all 
possessing  an  X  idiochromosome  or  two  such  elements  as  the  case 
may  be.  When,  therefore,  the  union  of  the  male  and  female 
pronuclei  takes  place  in  fertilization,  those  ova  that  are  fertilized 
by  a  spermatozoon  with  a  Y  idiochromosome  will  have  twenty- 


c^ 


o 


B 


c^ 


Fig.  17. — Diagrams  Illustrating  Sex-determination  in  Man,  A,  on  the 
Basis  of  the  Spermatogenesis  as  Described  by  Wieman,  B,  according  to 
Guyer's  Account. 

four  chromosomes,  two  of  which  are  the  X  and  Y  idiochromosomes, 
while  in  those  in  which  the  fertilization  is  accomplished  by  a 
spermatozoon  containing  an  X  idiochromosome,  there  will  also 
be  twenty-four  chromosomes,  but  two  of  these  will  be  X  idiochro- 
mosomes (Fig.  17,  ^) .  Or,  according  to  Guyer's  results,  one  group 
of  ova  will  have  twenty  ordinary  chromosomes  plus  four  idiochro- 


THE   FERTILIZATION    OF    THE    OVUM  35 

mosomes,  while  the  other  will  have  only  two  idiochromosomes 

(Fig.  17,^)- 

That  either  one  or  the  other  of  these  conditions  occurs  in  the 
fertilization  of  the  human  ovum  is  merely  a  conjecture  based  on 
what  has  been  shown  to  take  place  in  a  number  of  invertebrates 
and  most  clearly  in  insects.  In  these  two  classes  of  spermatozoa 
have  been  found  to  occur,  as  in  man,  the  classes  differing  in  some 
cases  in  the  number  and  in  others  in  the  quality  of  their  chromo- 
somes in  their  somatic  cells,  and  when  the  spermatozoal  differ- 
ence is  one  of  quality  in  the  chromosomes,  the  number  being 
identical,  it  may  be  supposed  that  this  difference  also  is  transmitted 
to  the  adults.  Further,  there  is  strong  evidence  that  those  indi- 
viduals that  develop  from  ova  having  the  larger  number  of  chromo- 
somes and  that  have  themselves  this  larger  number  in  their  somatic 
cells  are  females,  while  those  from  ova  with  the  smaller  number  of 
chromosomes  and  with  the  smaller  number  in  their  somatic  cells 
are  males;  where  the  difference  in  the  chromosomes  is  one  of 
quality  only  the  correlations  just  mentioned  are  not  so  readily 
perceived,  though  they  may  be  presumed  to  exist.  It  would  seem 
then  that  the  sex  of  a  given  individual  is  determined  by  the  pres- 
ence or  absence  or  quality  of  the  idiochromosomes  in  the  fertilized 
ovum  and  is  determined  at  the  time  of  that  fertilization.  While 
great  discrepancies  occur  in  the  various  descriptions  of  human 
spermatogenesis,  it  seems  probable  that  idiochromosomes  occur 
in  the  human  germ  cells  and  it  is  justifiable  to  attribute  to  them 
the  significance  that  has  been  so  definitely  shown  to  be  attached 
to  similar  structures  in  insects. 

It  seems  to  be  a  rule  that  but  one  spermatozoon  penetrates  the 
ovum.  Many,  of  course,  come  into  contact  with  it  and  endeavor 
to  penetrate  it,  but  as  soon  as  one  has  been  successful  in  its  en- 
deavor no  further  penetration  of  others  occurs.  The  reasons  for 
this  are  in  most  cases  obscure ;  experiments  on  the  ova  of  inverte- 
brates have  shown  that  the  subjection  of  the  ova  to  abnormal 
conditions  which  impair  their  vitality  favors  the  penetration  of 
more  than  a  single  spermatozoon  {polsy penny) ,  and,  indeed,  it 
appears  that  in  some  forms,  such  as  the  common  newt  (Diemy- 


36  THE    FERTILIZATION    OF   THE    OVUM 

ctylus),  polyspermy  is  the  rule,  only  one  of  the  spermatozoa,  how- 
ever, which  have  penetrated  uniting  with  the  female  pronucleus, 
the  rest  being  absorbed  by  the  cytoplasm  of  the  ovum. 

Fertilization  marks  the  beginning  of  development,  and  it  is 
therefore  important  that  something  should  be  known  as  to  where 
and  when  it  occurs.  It  seems  probable  that  in  the  human  species 
the  spermatozoa  usually  come  into  contact  with  the  ovum  and 
fertilize  it  in  the  upper  part  of  the  Fallopian  tubes,  and  the  occur- 
rence of  extra-uterine  pregnancy  (see  p.  23)  seems  to  indicate  that 
occasionally  the  ovum  may  be  fertilized  even  before  it  has  been 
received  into  the  tube. 

It  is  evident,  then,  that  when  fertilization  is  accomplished  the 
spermatozoon  must  have  traveled  a  distance  of  about  twenty-four 
centimeters,  the  length  o  the  upper  part  of  the  vagina  being  taken 
to  be  about  5  cm.,  that  of  the  uterus  as  7  cm.,  and  that  of  the  tube 
as  12  cm.  A  considerable  interval  of  time  is  required  for  the  com- 
pletion of  this  journey,  even  though  the  movement  of  the  sperma- 
tozoon be  tolerably  rapid.  The  observations  of  Henle  and  Hensen 
indicate  that  a  spermatozoon  may  progress  in  a  straight  line  at 
about  the  rate  of  from  1.2  to  2.7.  mm.  per  minute,  while  Lott  finds 
the  rate  to  be  as  high  as  3.6  mm.  Assuming  the  rate  of  progress 
to  be  about  2.5  mm.  per  minute,  the  time  required  by  the  spe 
matozoon  to  travel  from  the  upper  part  of  the  vagina  to  the  upper 
part  of  a  Fallopian  tube  will  be  about  one  and  a  half  hours  (Strass- 
mann).  This,  however,  assumes  that  there  are  no  obstacles  in 
the  way  of  the  rapid  progress  of  the  spermatozoon,  which  is  not  the 
case,  since,  in  the  first  place,  the  irregularities  and  folds  of  the 
lining  membrane  of  the  tube  render  the  path  of  the  spermatozoon 
a  labyrinthine  one,  and,  secondly,  the  action  of  the  cilia  of  the 
epithelium  of  the  tube  and  uterus  being  from  the  ostium  of  the 
tube  toward  the  os  uteri,  it  will  greatly  retard  the  progress; 
furthermore,  it  is  presumable  that  the  rapidity  of  movement  of  the 
spermatozoon  diminishes  after  a  certain  interval  of  time.  It 
seems  probable,  therefore,  that  fertilization  does  not  occur  for 
some  hours  after  coition,  even  providing  an  ovum  is  in  the  tube 
awaiting  the  approach  of  the  spermatozoon. 


SUPERFETATION  37 

But  this  condition  is  not  necessarily  present,  and  consequently 
the  question  of  the  duration  of  the  vitality  of  the  sperm  cell  be- 
comes of  importance.  Ahlfeld  has  found  that,  when  kept  at  a 
proper  temperature,  a  spermatozoon  will  retain  its  vitality  outside 
the  body  for  eight  days,  and  Diihrssen  reports  a  case  in  which 
living  spermatozoa  were  found  in  a  Fallopian  tube  removed  from 
a  patient  who  had  last  been  in  coitu  about  three  and  a  half  weeks 
previously.  As  regards  the  duration  of  the  vitality  of  the  ovum 
less  accurate  data  are  available.  Hyrtl  found  an  apparently 
normal  ovum  in  the  uterine  portion  of  the  left  tube  of  a  female 
who  died  three  days  after  the  occurrence  of  her  second  menstrua- 
tion, and  Issmer  estimates  the  duration  of  the  capacity  for  fertili- 
zation of  an  ovum  to  be  about  sixteen  days. 

It  is  evident,  then,  that  even  when  the  date  of  the  coitus  that 
led  to  fertilization  is  known,  the  actual  moment  of  the  latter  proc- 
ess and,  therefore,  the  exact  age  of  an  embryo,  can  only  be 
approximated  (see  p.  105).  For  the  determination  of  the  prob- 
able time  of  parturition  the  date  of  the  last  menstruation  is  in 
the  majority  of  cases  the  only  available  datum  and  the  statistics 
collected  by  Issmer  show  that  in  1220  cases  the  duration  of  preg- 
nancy averaged  280  days,  counting  from  the  first  day  of  the  last 
menstruation.  This  corresponds  to  ten  lunar  and  about  nine 
calendar  months,  but  an  estimate  on  this  basis  is  only  an  average 
from  which  considerable  variation  is  possible. 

Superfetation. — ^The  occasional  occurrence  of  twin  fetuses  in 
different  stages  of  development  has  suggested  the  possibility  of  the 
fertilization  of  a  second  ovum  as  the  result  of  a  coition  at  an  appreci- 
able interval  of  time  after  the  first  ovum  has  started  upon  its  devel- 
opment. There  seems  to  be  good  reason  for  believing  that  many  of 
the  cases  of  supposed  superfetation,  as  this  phenomenon  is  termed,  are 
instances  of  the  simultaneous  fertilization  of  two  ova,  one  of  which, 
for  some  cause  concerned  with  the  supply  of  nutrition,  has  later  failed 
to  develop  as  rapidly  as  the  other.  At  the  same  time,  however,  even 
although  the  phenomenon  may  be  of  rare  occurrence,  it  is  by  no 
means  impossible,  for  occasionally  a  second  Graafian  follicle,  either  in 
the  same  or  the  other  ovary,  may  be  so  near  maturity,  that  its  ovum 
is  extruded  soon  after  the  first  one,  and  if  the  development  of  the 
latter  and  the  incidental  changes  in  the  uterine  mucous  membrane 
have   not  proceeded  so  far   as  to  prevent  the   access   of  the  sper- 


3^  LITERATURE 

matozoon  to  the  ovum,  its  fertilization  and  development  may  ensue. 
The  changes,  however,  which  prevent  the  passage  of  the  spermatozoon 
are  completed  early  in  development  and  the  difference  between  the 
normally  developed  embryo  and  that  due  to  superfetation  will  be 
comparatively  small,  and  will  become  less  and  less  evident  as  devel- 
opment proceeds,  provided  that  the  supply  of  nutrition  to  both 
embryos  is  equal. 

LITERATURE 

E.  Ballowitz:  "Untersuchungen  uber  die  Struktur  der  Spermatozoen,"  No.  4. 

Zeitschr.  fur  wissensch.  Zool.,  lii,  1891. 
K.  VON  Bardeleben:  "Beitrage  zur  Histologic  des  Hodens  und  zur  Spermatogenese 

beim  Menschen,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Ahth.,  Supplement,  1897. 
Th.  Boveri:  " Befruchtung,"  Ergehnisse  der  Anat.  und  Entwicklungsgesch,.  1,  1892. 
J.  G.  Clark:  "Ursprung,  Wachsthum  und  Ende  des  Corpus  luteum  nach  Beobach- 

tungen  am  Ovarium  des  Schweines  und  des  Menschen,"  Archiv  fiir  Anat.  und 

Physiol.,  Anat.  Ahth.,  1898. 
G.  W.  Corner:  "The  corpus  luteum  of  pregnancy  in  Swine."     Contrib.  on  Emhryol. 

ii,  Carnegie  Inst.  Publ.  222, 191 5. 
D.  Drips:  "Studies  on  the  ovary  of  the  Spermophile  (Spermophilus  citnlus  tridecim- 

lineatus)  with  special  reference  to  the  corpus  luteum,"  Amer.  Jour.  Anat.,  xxv, 

1919. 
L.  Fraenkel:  "Neue  Experimente  zur  Function  des  Corpus  luteum,"  Arch,  fur 

Gynaek.fXd,  19 10. 
L.  Fraenkel:  "Das  zeitliche  Verhalten  von  Ovulation  und  Menstruation,"  Zenlralhl. 

fUrGynaek.,  191 1. 
L.  Fraenkel:  "Ovulation,  Konzeption    und    Schwangerschaftsdauer,"  Zeit.  fur 

Gehurtsh.  u.  Gynaek.,  lxxiv,  19 13. 
L.  Gerlach:  "Ueber  die  Bildung  der  Richtungskorper  bei  Mus  musculus,"  Wies- 
baden, 1906. 
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und  Menstruationstermin,"  Anat.  Anz,XLVU,  1914. 
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in  der  Spermiogenese  des  Menschen,"  Arch.  f.  mikr.  Anat.,  lxxex,  1912. 
M.  F.  Guyer:  "Accessory  Chromosomes  in  Man,"  Biol.  Bull.,  XJX,  1910. 
W.  Heape:  "The  Sexual  Season  of  Mammals  and  the  Relation  of  the  Preoestrum  to 

Menstruation,"  Quart.  Journ.  Micros.  Sci.,  N.  S.,  xliv,  1901  (contains  very  full 

bibliography). 
O.  Hertwig:  "Vergleich  der  Ei-  und  Samenbildung  bei  Nematoden,"  Archiv  fiir 

mikrosk.  Anat.,XKXVi,  1890. 

F.  HiTSCHMANN  and  L.  Adler:  "Der  Bau  der  Uterusschleimhaut  des  geschlects- 

reifen  Weibes,  mit  besonderer  Berucksichtigung  der  Menstruation,"  Monatsschr. 

fiir  Gehurtsh.  und  Gynaek.,  xxxii,  1908. 
J.  Janowski:  "Beitrag  zur  Entstehung  des  Corpus  luteum  der  Saugetiere,"  Arch.f. 

mikr.  Anat.,  Lxrv,  1904. 
W.  B.  Kirkham:  "The  Maturation  of  the  Mouse  Egg,"  Bioi,  Bulletin^  xii,  1907. 


LITERATURE  39 

W.  B.  KiRKHAM  and  H.  S.  Burr:  "The  breeding  habits,  maturation  of  eggs  and 

ovulation  of  the  albino  rat,"  Amer.  Jour.  Anat.,  xv,  1913. 
H.  Lams  and  J.  Doorme:  "Nouvelles  recherches  sur  la  maturation  et  la  feconda- 

tion  de  I'oeuf  de  mammiferes,"  Arch,  de  Biol.,  xxiii,  1907. 
H.  Lams:  "Etude  de  I'oeuf  de  Cobaye  aux  premiers  stades  de  I'embryogen^e,"- 

Arch,  de  Biol.,  xxviii,  1913. 
M.  VON  Lenhossek:  "Untersuchungen  uber  Spermatogenese,"  Archiv  filr  mikrosk. 

Anat.,  LI,  1898. 
G.  Leopold  and  A.  Rovano:  "Neuer  Beitrag  zur  Lehre  von  der  Menstruation  und 

Ovulation,"  Arch,  fiir  Gynaek.,  lxxxiii,  1907. 
W.  H.  Longley:  "The  Maturation  of  the  Egg  and  Ovulation  in  the  Domestic  Cat," 

Amer.  Journ.  Anat.,  xii,  191 1. 
F.  H.  A.  Marshall:  "The  (Estrus  Cycle  and  the  Formation  of  the  Corpus  luteum 

in  the  Sheep,"  PMo^,  Trans.,  Ser.  B,  cxcvi,  1904. 
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Journ.  Micros.  Sci.,  N.  S.,  xlix,  1906. 
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xciii,  191 1. 
R.  Mayer :  "Ueber  die  Beziehung  der  Eizelle  und  des befruchteten  Eies zutn  FoUikel- 

apparat,  sowie  des  Corpus  luteum  zum  Menstruation,"  Arch,  fiir  Gynaek,  c, 

1913- 

F.  Meves:  "Ueber  Struktur  und  Histogenese  der  Samenfaden  des  Meerschwein- 

chens,"  Archiv  fiir  mikrosk.  Anat.,  liv,  1899. 
J.  W.  Miller:     "Corpus  luteum.  Menstruation  und  Graviditat,"  Arch,  fiir  Gynaek, 

CI,  1914. 
T.  H.  Montgomery:  "Differentiation  of  the  human  Cells  of  Sertoli,"  Biolog.  Bull, 

XXI,  1911. 
T.  H.  Montgomery:  "Human  Spermatogenesis,  Spermatocytes,  and  Spermiogene- 

sis:  A  study  in  Inheritance,"  Jour.  Acad.  Nat.  Sci.,  Phila.,  Ser.  2,  xv,  1912. 
W.  Nagel:  "Das  menschliche  Ei,"  Archiv  fiir  mikrosk.  Anat.,  xxxi,  1888. 

G.  Niessing:  "Die  Betheiligung  der  Centralkorper  und  Sphare  am  Aufbau  des 

Samenfadens  bei  Saugethieren,"  Archiv  fiir  mikrosk..  Anat.,  XLVin,  1896. 
G.  Retzius:  "Die  Spermien  des  Menschen,"  Biolog.  Untersuch.,xiv,  1909. 
W.    Rubaschkin:  "Ueber    die    Reifungs-  und    Befruchtungs-processe  des    Meer- 

schweincheneies,"  Anat.  Hefte,  xxrx,  1905. 
C.  Ruge  II:  "Ueber  Ovulation,  Corpus  luteum  und  Menstruation,"  Arch,  fiir 

Gynaek.,  c,  19 13. 
S.  S.  Schochet:  "A  suggestion  as  to  the  process  of  ovulation  and  ovarian  cyst 

formation,"  Anat.  Record,  x,  1916. 
J.  Sobotta:  "Die  Befruchtung  und  Furchung  des  Eies  der  Maus,"  Archiv  fUr  mik- 
rosk. Anat.,  XLV,  1895. 
J.  Sobotta:  "Ueber  die  Bildung  des  Corpus  luteum  bei  der  Maus,"  Archiv  fiir 

mikrosk.  Anat.,  xlvii,  1897. 
J.  Sobotta:  "Ueber  die  Bildung  des  Corpus  luteum  beim  Meerschweinchen,"  Anat., 

Hefte,  xxxii,  1906. 
J.  Sobotta  and  G.  Burckhard:  "Reifung  und  Befruchtung  der  Eier  des  weissen 

Ratte,"  Anat.  Hefte,  xlu,  1910. 


40  LITERATURE 

P.  Strassmann  :  "  Beitrage  zur  Lehre  von  der  Ovulation,  Menstruation  und  Con- 
ception," Archiv  fiir  Gynaekol,  Lii,  1896. 

O.  Van  der  Stricht:  "Sur  le  processus  de  I'excretion  des  glandes  endocrines,  le 
corps  jaune  et  la  glande  interstitielle  de  I'ovaire;"  Arch,  de  Biol,  xxvii,  191 2. 

F.  Villemin:  "Le  corps  jaune  considere  comme  glande  a.  secretion  interne,"  Paris, 
1908. 

H.  VON  Winiwarter:  "Etudes  sur  la  spermatogenese  humaine,"  Arch,  de  Biol., 
XXVII,  1912. 

W.  Waldeyer:  "Eierstock  und  Ei,"  Leipzig,  1870. 

H.  L.  Wieman:  "The  chromosomes  of  human  spermatocytes,"  Anier.  Journ., 
Anat.,  XXI,  191 7. 


CHAPTER  II 

THE  SEGMENTATION  OF  THE  OVUM  AND  THE 
FORMATION  OF  THE  GERM  LAYERS 

Segmentation. — The  union  of  the  male  and  female  pronuclei 
has  already  been  described  as  being  accompanied  by  the  formation 
of  a  mitotic  spindle  which  produces  a  division  of  the  ovum  into 
two  cells.  This  first  division  is  succeeded  at  more  or  less 
regular  intervals  by  others,  until  a  mass  of  cells  is  produced  in 
which  a  differentiation  eventually  appears.  These  divisions  of 
the  ovum  constitute  what  is  termed  its  segmentation. 

The  mammalian  ovum  has  behind  it  a  long  line  of  evolution, 
and  even  at  early  stages  in  its  development  it  exhibits  peculiarities 
which  can  be  reasonably  explained  only  as  an  inheritance  of  past 
conditions.  One  of  the  most  potent  factors  in  modifying  the  char- 
acter of  the  segmentation  of  the  ovum  is  the  amount  of  food  yolk 
which  it  contains,  and  it  seems  to  be  certain  that  the  immediate 
ancestors  of  the  mammalia  were  forms  whose  ova  contained  a  con- 
siderable amount  of  yolk,  many  of  the  peculiarities  resulting  from 
its  presence  being  s.till  clearly  indicated  in  the  early  development 
of  the  almost  yolkless  human  ovum.  To  give  some  idea  of 
the  peculiarities  which  result  from  the  presence  of  considerable 
amounts  of  yolk  it  will  be  well  to  compare  the  processes  of  segmen- 
tation and  differentiation  seen  in  ova  with  different  amounts  of  it. 

A  little  below  the  scale  of  the  vertebrates  proper  is  a  form, 
Amphioxus,  which  possesses  an  almost  yolkless  ovum,  presenting 
a  simple  process  of  development.  The  fertilized  ovum  of  Amphi- 
oxus in  its  first  division  separates  into  two  similar  and  equal  cells, 
and  these  are  made  four  (Fig.  i8,  A)  by  a  second  plane  of  division 
which  cuts  the  previous  one  at  right  angles.  A  third  plane  at 
right  angles  to  both  the  preceding  ones  brings  about  an  eight- 

41 


42 


THE    SEGMENTATION    OF   THE    OVUM 


celled  stage  (Fig.  i8,  B),  and  further  divisions  result  in  the  forma- 
tion of  a  large  number  of  cells  which  arrange  themselves  in  the 
form  of  a  hollow  sphere  which  is  known  as  a  hlastula  (Fig.  i8,  £). 
The  minute  amount  of  yolk  which  is  present  in  the  ovum  of 
Amphioxus  collects  at  an  early  stage  of  the  segmentation  at  one 
pole  of  the  ovum,  the  cells  containing  it  being  somewhat  larger 
than  those  of  the  other  pole  (Fig.  i8,  B),  and  in  the  bias  tula 
the  cells  of  one  pole  are  larger  and  more  richly  laden  with  yolk 
than  those  of  the  other  pole  (Fig.  i8,  F).  If,  now,  the  segmenting 
ovum  of  an  Amphibian  be  examined,  it  will  be  found  that  a  very 


Fig.  1 8. — Stages  in  the  Segmentation  of  Amphioxus. 

A,  Four-celled  stage;  B,  eight-celled  stage;  C,  sixteen-celled  stage;  D,  early  blastuh 

E,  blastula;  F,  section  of  blastula. — (Hatschek.) 


much  greater  amount  of  yolk  is  present  and,  as  in  Amphioxus,  it 
is  located  especially  at  one  pole  of  the  ovum.  The  first  three 
planes  of  segmentation  have  the  same  relative  positions  as  in 
Amphioxus  (Fig.  i8),  but  one  of  the  tiers  of  cells  of  the  eight-celled 
stage  is  very  much  smaller  than  the  other  (Fig.  19,  B).  In  the 
subsequent  stages  of  segmentation  the  small  cells  of  the  upper 
pole  divide  more  rapidly  than  the  larger  ones  of  the  lower  pole, 
the  activity  of  the  latter  seeming  to  be  retarded  by  the  accumula- 
tion of  the  yolk,  and  the  resulting  blastula  (Fig.  19,  D)  shows  a 
very  decided  difference  in  the  size  of  the  cells  of  the  two  poles. 


THE    SEGMENTATION    OF    THE    OVUM 


43 


In  the  ova  of  reptiles  and  birds  the  amount  of  yolk  stored  up 
in  the  ovum  is  very  much  greater  than  in  the  amphibia,  and 
it  is  aggregated  at  one  pole  of  the  ovum,  of  which  it  forms  the 
principal  mass,  the  yolkless  protoplasm  appearing  as  a  small  disk 
upon  the  surface  of  a  relatively  huge  mass  of  yolk.  The  inertia 
of  this  mass  of  nutritive  material  is  so  great  that  the  segmentation 
is  confined  to  the  small  yolkless  disk  of  protoplasm  and  affects 


^^^^ 


C  D 

Fig.  19. — Stages  in  the  Segmentation  of  Amhlysioma. — {Eycleshymer.) 

consequently  only  a  portion  of  the  entire  ovum.  To  distinguish 
this  form  of  segmentation  from  that  which  affects  the  entire  ovum 
it  is  termed  merohlastic  segmentation,  the  other  form  being  known 
as  holoblastic. 

In  the  ovum  of  a  turtle  or  a  bird  the  first  plane  of  segmentation 
crosses  the  protoplasmic  disk,  dividing  it  into  two  practically 
equal  halves,  and  the  second  plane  forms  at  approximately  right 
angles  to  the  first  one,  dividing  the  disk  into  four  quadrants  (Fig. 


44 


THE    SEGMENTATION    OF   THE    OVUM 


20,  A).  The  third  division,  like  the  two  which  precede  it,  is  radial 
in  position,  while  the  fourth  is  circular  and  cuts  off  the  inner  ends 
of  the  six  cells  previously  formed  (Fig.  20,  C).  The  disk  now  con- 
sists of  six  central  smaller  cells  surrounded  by  six  larger  peripheral 
ones.  Beyond  this  period  no  regularity  can  be  discerned  in  the 
appearance  of  the  segmentation  planes;  but  radial  and  circular 
divisions   continuing  to  form,  the  disk  becomes  divided  into  a 


I) 


Fig.    20. — Four   Stages   in   the   Segmentation   of   the   Blastoderm   of  the 

Chick. — iCoste.) 

large  number  of  cells,  those  at  the  center  being  much  smaller  than 
those  at  the  periphery.  In  the  meantime,  however,  the  smaller 
central  cells  have  begun  to  divide  in  planes  parallel  to  the  surface 
of  the  disk,  which,  from  being  a  simple  plate  of  cells,  thus  becomes 
a  discoidal  cell-mass. 

During  the  segmentation  of  the  disk  it  has  increased  materially 
in  size,  extending  further  and  further  over  the  surface  of  the  yolk, 
into  the  substance  of  which  some  of  the  lower  cells  of  the  discoidal 


THE    SEGMENTATION    OF    THE    OVUM  45 

cell-mass  have  penetrated.  A  comparison  of  the  diagram  (Fig. 
2i)  of  the  ovum  of  a  reptile  at  about  this  stage  of  development 
with  the  figure  of  the  amphibian  bias  tula  (Fig.  19,  D)  will  indicate 
the  similarity  between  the  two,  the  large  yolk-mass  (F)  of  the" 
reptile  with  the  scattered  cells  which  it  contains  corresponding  to 
the  lower  pole  cells  of  the  amphibian  blastula,  the  central  cavity 
of  which  is  practically  suppressed  in  the  reptile.  Beyond  this 
stage,  however,  the  similarity  becomes  more  obscured.  The  peri- 
pheral cells  of  the  disk  continue  to  extend  over  the  surface  of 
the  yolk  and  finally  completely  enclose  it,  forming  an  enveloping 


Fig.  21. — Diagram  Illustrating  a  Section  of  the  Ovum  of  a  Reptile  at  a 

Stage  Corresponding  to  the  Blastula  of  an  Amphibian. 

hi.  Blastoderm;  Y,  yolk-mass. 

layer  which  is  completed  at  the  upper  pole  of  the  egg  by  the  dis- 
coidal  cell-mass,  or,  as  it  is  usually  termed,  the  blastoderm. 

Turning  now  to  the  mammalia,*  it  will  be  found  that  the  ovum 
in  the  great  majority  is  almost  or  quite  as  destitute  of  food  yolk 
as  is  the  ovum  of  Amphioxus,  with  the  result  that  the  segmenta- 
tion is  of  the  total  or  holoblastic  type.  It  does  not,  however, 
proceed  with  that  regularity  which  marks  the  segmentation  of 
Amphioxus  or  an  amphibian,  but  while  at  first  it  divides  into 
two  slightly  unequal  cells  (Fig.  22),  thereafter  the  divisions  be- 

*  The  segmentation  of  the  human  ovum  has  not  yet  been  observed;  what  follows 
is  based  on  what  occurs  in  the  ovum  of  the  rabbit,  mole,  and  especially  of  a  bat  (Van 
Beneden) . 


46 


THE    SEGMENTATION    OF    THE    OVUM 


come  irregular,  three-celled,  four-celled,  five-celled,  and  six-celled 
stages  having  been  observed  in  various  instances.  Nor  is  the 
result  of  the  final  segmentation  a  hollow  vesicle  or  blastula,  but  a 
solid  mass  of  cells,  termed  a  morula,  is  formed.  This  structure 
is  not,  however,  comparable  to  the  blastula  of  the  lower  forms,  but 


Fig.  22. — Pour  Stages  in  the  Segmentation  of  the  Ovum  of  a  Mouse. 
X,  Polar  globule. — (Sobotta.) 


corresponds  to  a  stage  of  reptilian  development  a  little  later  than 
that  shown  in  Fig.  21,  since,  as  will  be  shown  directly,  the  cells 
corresponding  to  the  blastoderm  and  the  enveloping  layer  are 
already  present.  There  is,  then,  no  blastula  stage  in  the  mam- 
malian development 

Differentiation  now  begins  by  the  peripheral  cells  of  the  morula 
becoming  less  spherical  in  shape  and  later  forming  a  layer  of  flat- 
tened cells,  the  enveloping  layer,  surrounding  the  more  spherical 


THE    SEGMENTATION   OF   THE    OVUM 


47 


central  cells  (Fig.  23,  A).     In  the  latter  vacuoles  now  make  their 
appearance,  especially  in  those  cells  which  are  nearest  what  may 


C  D 

Fig.  23. — Later  Stages  in  the  Segmentation  of  the  Ovum  of  a  Bat. 
A.  C,  and  D  are  sections,  B  a  surface  view. — (Van  Beneden.) 

be  regarded  as  the  lower  pole  of  the  ovum  (Fig.  22,  C)  and  these 
vacuoles,  gradually  increasing  in  size,  eventually  become  confluent, 


48       TWIN  DEVELOPMENT  AND  DOUBLE  MONSTERS 

the  condition  represented  in  Fig.  23,  D,  being  produced.  At  this 
stage  the  ovum  consists  of  an  enveloping  layer,  enclosing  a  cavity 
which  is  equivalent  to  the  yolk-mass  of  the  reptilian  ovum,  the 
vacuolization  of  the  inner  cells  of  the  morula  representing  a  belated 
formation  of  yolk.  On  the  inner  surface  of  the  enveloping  layer, 
at  what  may  be  termed  the  upper  pole  of  the  ovum,  is  a  mass  of 
cells  projecting  into  the  yolk-cavity  and  forming  what  is  known 
as  the  inner  cell-mass,  a  structure  comparable  to  the  blastoderm 
of  the  reptile.  In  one  respect,  however,  a  difference  obtains,  the 
inner  cell-mass  being  completely  enclosed  within  the  enveloping 
cells,  which  is  not  the  case  with  the  blastoderm  of  the  reptile. 
That  portion  of  the  enveloping  layer  which  covers  the  cell-mass 
has  been  termed  Rauber^s  covering  layer,  and  probably  owes  its 
existence  to  the  precocity  of  the  formation  of  the  enveloping  layer. 
It  is  clear,  then,  that  an  explanation  of  the  early  stages  of 
development  of  the  mammalian  ovum  is  to  be  obtained  by  a  com- 
parison, not  with  a  yolkless  ovum  such  as  that  of  Amphioxus,  but 
with  an  ovum  richly  laden  with  yolk,  such  as  the  meroblastic 
ovum  of  a  reptile  or  bird.  In  these  forms  the  nutrition  necessary 
for  the  growth  of  the  embryo  and  for  the  complicated  processes 
of  development  is  provided  for  by  the  storing  up  of  a  quantity 
of  yolk  in  the  ovum,  the  embryo  being  thus  independent  of  external 
sources  for  food.  The  same  is  true  also  of  the  lowest  mammalia, 
the  Monotremes,  which  are  egg-laying  forms  producing  ova 
resembling  greatly  those  of  a  reptile.  When,  however,  in  the 
higher  mammals  the  nutrition  of  the  embryo  became  provided 
for  by  the  attachment  of  the  embryo  to  the  walls  of  the  uterus 
of  the  parent  so  that  it  could  be  nourished  directly  by  the  parent, 
the  storing  up  of  yolk  in  the  ovum  was  unnecessary  and  it  became 
a  holoblastic  ovum,  although  many  peculiarities  dependent  on  the 
original  meroblastic  condition  persisted  in  its  development. 

Twin  Development. — ^As  a  rule,  in  the  human  species  but  one 
embryo  develops  at  a  time,  but  the  occurrence  of  twins  is  by  no 
means  infrequent,  and  triplets  and  even  quadruplets  occasinoally  are 
developed.  The  occurrence  of  twins  may  be  due  to  two  causes,  either 
to  the  simultaneous  ripening  and  fertilization  of  two  ova,  either  from 
one  or  from  both  ovaries,  or  to  the  separation  of  a  single  fertilized 


TWIN  DEVELOPMENT  AND  DOUBLE  MONSTERS        49 

ovum  into  two  independent  parts  during  the  early  stages  of  develop- 
ment. That  twins  may  be  produced  by  this  latter  process  has  been 
abundantly  shown  by  experimentation  upon  developing  ova  of  lower 
forms,  each  of  the  two  cells  of  an  Amphioxus  ovum  in  that  stage  of 
development,  if  mechanically  separated,  completing  its  development' 
and  producing  an  embryo  of  about  half  the  normal  size.  Furthermore, 
it  has  been  shown  (Patterson)  that  in  the  armadillo  a  division  of 
the  embryonic  anlage  into  four  parts  at  an  early  stage  of  the  develop- 
ment is  a  normal  process,  the  four  young  produced  at  a  birth  being 
quadruplets  produced  from  a  single  ovum. 

It  is  noteworthy  that  in  the  case  of  the  armadillo  the  four  indi- 
viduals of  each  birth  are  of  the  same  sex,  and  it  is  probable  that  human 
twins  of  the  same  sex  and  closely  similar  in  appearance,  what  are 
termed  "like"  twins,  are  the  result  of  a  division  of  a  single  embryonic 
anlage,  while  "unlike"  twins  are  produced  by  the  simultaneous  fer- 
tilization of  two  separate  ova. 

Double  Monsters  and  the  Duplication  of  Parts. — The  occasional 
occurrence  of  double  monsters  is  explained  by  an  imperfect  separation 
into  two  parts  of  an  originally  single  embryo,  the  extent  of  the  separa- 
tion, and  probably  also  the  stage  of  development  at  which  it  occurs, 
determining  the  amount  of  fusion  of  the  two  individuals  constituting 
the  monster.  All  gradations  of  separation  occur,  from  almost  complete 
separation,  as  seen  in  such  cases  as  the  Siamese  twins,  to  forms  in 
which  the  two  individuals  are  unite  throughout  the  entire  length  of 
their  bodies.  The  separation  may  also  afiFect  only  a  portion  of  the 
embryo,  producing,  for  instance,  double-faced  or  double-headed  mon- 
sters or  various  forms  of  so-called  parasitic  monsters;  and  finally,  it 
may  affect  only  a  group  of  cells  destined  to  from  a  special  organ, 
producing  an  excess  of  parts,  such  as  supernumerary  digits  or  accessory 
spleens. 

It  has  been  observed  in  the  case  of  double  monsters  that  one  of  the 
two  fused  individuals  always  has  the  position  of  its  various  organs 
reversed,  it  being,  as  it  were,  the  looking-glass  image  of  its  fellow. 
Cases  of  a  similar  situs  inversus  visceruntj  as  it  is  called,  have  not  in- 
frequently been  observed  in  single  individuals,  and  a  plausible  ex- 
planation of  such  cases  regards  them  as  one  of  a  pair  of  twins  formed 
by  the  incomplete  division  of  a  single  embryo,  the  other  individual 
having  ceased  to  develop  and  either  having  undergone  degeneration  or 
having  been  included  within  the  body  of  the  apparently  single  indivi- 
dual. Another  explanation  of  situs  inversus  has  been  advanced  (Con- 
klin)  on  the  basis  of  what  has  been  observed  in  certain  invertebrates. 
In  some  species  of  snails  situs  inversus  is  a  noimal  condition  and  it  has 
been  found  that  the  inversion  may  be  traced  back  in  the  development 
even  to  the  earliest  segmentation  stages.  The  conclusion  is  thereby 
indicated  that  its  primary  cause  may  reside  in  an  inversion  of  the 
polarity  of  the  ovum,  evidence  being  forthcoming  in  favor  of  the  view 
that  even  in  the  ovum  of  these  and  other  forms  there  is  probably  a 


50  FORMATION    OF    THE    GERM    LAYERS 

distinct  polar  differentiation.  How  far  this  view  may  be  applicable  to 
the  mammalian  ovum  is  uncertain,  but  if  it  be  applicable  it  explains 
the  phenomenon  of  inversion  without  complicating  it  with  the  question 
of  twin-formation. 

The  Formation  of  the  Germ  Layers. — During  the  stages  which 
have  been  described  as  belonging  to  the  segmentation  period  of 
development  there  has  been  but  little  differentiation  of  the  cells. 
In  Amphioxus  and  the  amphibians  the  cells  at  one  pole  of  the 
bjastula  are  larger  and  more  yolk-laden  than  those  at  the  other 
pole,  and  in  the  mammals  an  inner  cell-mass  can  be  distinguished 
from  the  enveloping  cells,  this  latter  differentiation  having  been 


A  B 

Pig.  24. — Two  Stages  in  the  Gastrulation  of  Amphioxus. — {Morgan  and  Hazen.) 

anticipated  in  the  reptiles  and  being  a  differentiation  of  a  portion 
of  the  ovum,  from  which  alone  the  embryo  will  develop,  from  a 
portion  which  will  give  rise  to  accessory  structures.  In  later 
stages  a  differentiation  of  the  inner  cell-mass  occurs,  resulting  first 
of  all  in  the  formation  of  a  two-layered  or  diplohlastic  and  later  of  a 
three-layered  or  triplohlastic  stage. 

Just  as  the  segmentation  has  been  shown  to  be  profoundly 
modified  by  the  amount  of  yolk  present  in  the  ovum  and  by  its 
secondary  reduction,  so,  too,  the  formation  of  the  three  primitive 
layers  is  much  modified  by  the  same  cause,  and  to  get  a  clear 
understanding  of  the  formation  of  the  triplohlastic  condition  of 
the  mammal  it  will  be  necessary  to  describe  briefly  its  develop- 
ment in  lower  forms. 


FORMATION  OF  THE  GERM  LAYERS 


51 


In  Amphioxus  the  diploblastic  condition  results  from  the  flat- 
tening of  the  large-celled  pole  of  the  blastula  (Fig.  24,  A),  and 
finally  from  the  invagination  of  this  portion  of  the  vesicle  within 
the  other  portion  (Fig.  24,  B).  The  original  single-walled  blastula 
in  this  way  becomes  converted  into  a  double-walled  sac  termed  a 
gastrula,  the  outer  layer  of  which  is  known  as  the  ectoderm  or 
epihlast  and  the  inner  layer  as  the  endoderm  or  hypoblast.  The 
cavity  bounded  by  the  endoderm  is  the  primitive  gut  or  archen- 
teron,  and  the  opening  by  which  this  communicates  with  the  ex- 
terior is  the  blastopore.  This  last 
structure  is  at  first  a  very  wide 
opening,  but  as  development  pro- 
ceeds it  becomes  smaller,  and 
finally  is  a  relatively  small  open- 
ing situated  at  the  posterior  ex- 
tremity of  what  will  be  the  dorsal 
surface  of  the  embryo. 

As  the  oval  embryo  continues 
to  elongate  in  its  later  develop- 
ment the  third  layer  or  mesoderm 
makes  its  appearance.  It  arises 
as  a  lateral  fold  (mp)  of  the  dorsal 


Fig.  25. — Transverse  Section  of 
A  mphioxus  Embryo  with  Five 
Mesodermic  Pouches. 

Ch,  Notochord;  d,  digestive  cavity; 
surface    of    the    endoderm    (en)    on    ^c,  ectoderm;   en,  endoderm;  m,  medul- 
1       •J         r  ,1  -jji     T  •         lary  plate;    mp,  mesodermic  pouch. — 

each  side  of  the  middle  line  as  m-   (Hatschek.) 
dicated  in  the  transverse  section 

shown  in  Fig.  25.  This  fold  eventually  becomes  completely  con- 
stricted off  from  the  endoderm  and  forms  a  hollow  plate  oc- 
cupying the  space  between  the  ectoderm  and  endoderm,  the  cavity 
which  it  contains  being  the  body-cavity  or  ccelom. 

In  the  amphibia,  where  the  amount  of  yolk  is  very  much  greater 
than  in  Amphioxus,  the  gastrulation  becomes  considerably  modi- 
fied. On  the  line  where  the  large-  and  small-celled  portions  of 
the  blastula  become  continuous  a  crescentic  groove  appears  and, 
deepening,  forms  an  invagination  (Fig.  26,  gc),  the  roof  of  which  is 
composed  of  relatively  small  yolk-containing  cells  while  its  floor 
is  formed  by  the  large  cells  of  the  lower  pole  of  the  blastula.     The 


52 


FORMATION    OF    THE    GERM    LAYERS 


cavity  of  the  blastula  is  not  sufficiently  large  to  allow  of  the  typical 
invagination  of  all  these  large  cells,  so  that  they  become  enclosed 
by  the  rapid  growth  of  the  ectoderm  cells  of  the  upper  pole  of  the 
ovum  over  them.  Before  this  growth  takes  place  the  blastopore 
corresponds  to  the  entire  area  occupied  by  the  large  yolk  cells, 
but  later,  as  the  growth  of  the  smaller  cells  gradually  encloses  the 
larger  ones,  it  becomes  smaller  and  is  finally  represented  by  a 


Pig.  26. — Section  through  a  Gastrula  of  Ambly stoma, 
dl,  Dorsal  lip  of  blastopore;  gc,  digestive  cavity;  gr,  area  of  mesoderm  formation;  mes, 
mesoderm. — (Eycleshymer.) 

small  opening  situated  at  what  w'll  be  the  hind  end  of  the  embryo. 
Soon  after  the  archenteron  has  been  formed  a  solid  plate  of  cells, 
eventually  splitting  into  two  layers,  arises  from  its  roof  on  each  side 
of  the  median  line  and  grows  out  into  the  space  between  the 
ectoderm  and  endoderm  (Fig.  27,  mk^  and  mk^),  evidently 
corresponding  to  the  hollow  plates  formed  in  the  same  situations 
in  Amphioxus.  This  is  not,  however,  the  only  source  of  the 
mesoderm  in  the  amphibia,  for  while  the  blastopore  is  still  quite 
large  there  may  be  found  surrounding  it,  between  the  endoderm 
and  ectoderm,  a  ring  of  mesodermal  tissue  (Fig.  26,  mes).    As  the 


FORMATION  OF  THE  GERM  LAYERS 


53 


blastopore  diminishes  in  size  and  its  lips  come  together  and 
unite,  the  ring  of  mesoderm  forms  first  an  oval  and  then  a  band 
lying  beneath  the  line  of  closure  of  the  blastopore  and  united  with 
both  the  superjacent  ectoderm  and  the  subjacent  endoderm. 
This  line  of  fusion  of  the  three  germ  layers  is  known  as  the 
primitive  streak.  It  is  convenient  to  distinguish  the  mesoderm  of 
the  primitive  streak  from  that  formed  from  the  dorsal  wall  of  the 
archenteron  by  speaking  of  the  former  as  the  prostomial  and  the 
latter  as  the  gastral  mesoderm,  though  it  must  be  understood  that 


Fig.  27. — Section  through  an  Embryo  Amphibian  (Triton)  of  2%  Days,  show- 
ing THE  Formation  of  the  Gastral  Mesoderm. 

ak,  Ectoderm;  ch,  chorda  endoderm;  dk,  digestive  cavity;  ik,  endoderm;  mk^  and 
mk"^,  somatic  and  splanchnic  layers  of  the  mesoderm.  D,  dorsal  and  F,  ventral. — 
{Her  twig.) 

the  two  are  continuous  immediately   in  front  of  the   definitive 
blastopore. 

In  the  reptilia  still  greater  modifications  are  found  in  the 
method  of  formation  of  the  germ  layers.  Before  the  enveloping 
cells  have  completely  surrounded  the  yolk-mass,  a  crescen tic  groove, 
resembling  that  occurring  in  amphibia,  appears  near  the  posterior 
edge  of  the  blastoderm  the  cells  of  which,  in  front  of  the  groove, 
arrange  themselves  in  a  superficial  layer  one  cell  thick,  which  may 
be  regarded  as  the  ectoderm  (Fig.  28,  ec),  and  a  subjacent  mass  of 
somewhat  scattered  cells.  Later  the  lowermost  cells  of  this  sub- 
jacent mass  arrange  themselves  in  a  continuous  layer,  constituting 


54 


rORMATION    OF    THE    GERM    LAYERS 


what  is  termed  the^ primary  endoderm  (en^),  while  the  remaining 
cells,  aggregated  especially  in  the  region  of  the  crescentic  groove, 
form  the  prostomial  mesoderm  {prm).  In  the  region  enclosed  by 
the  groove  a  distinct  delimitation  of  the  various  layers  does  not 
occur,  and  this  region  forms  the  primitive  streak.  The  groove 
now  begins  to  deepen,  forming  an  invagination  of  secondary  en- 
doderm, the  extent  of  this  invagination  being,  however,  very 
different  in  different  species.     In  the  gecko  (Will)  it  pushes  for- 


•ei^ikmt®     ^^ 


prm 


^"?3E1^ 


ec 
en 


Fig.  28. — Longitudinal  Sections  through  Blastoderms  of  the  Gecko,  showing 

Gastrulation. 
ec,   Ectoderm;   en,   secondary   endoderm;   en\  primary  endoderm;   prm,  prostomial 

mesoderm. — {Will.)      ^ 

ward  between  the  ectoderm  and  primary  endoderm  almost  to  the 
anterior  edge  of  the  blastoderm  (Fig.  28,  B),  but  later  the  cells 
forming  its  floor,  together  with  those  of  the  primary  endoderm 
immediately  below,  undergo  a  degeneration,  the  roof  cells  at  the 
tip  and  lateral  margins  of  the  invagination  becoming  continuous 
with  the  persisting  portions  of  the  primary  endoderm  (Figs.  28,  C 
and  29,  B).  This  layer,  following  the  enveloping  cells  in  their 
growth  over  the  yolk-mass,  gradually  surrounds  that  structure  so 


FORMATION    OF    THE    GERM   LAYERS 


55 


that  it  comes  to  lie  within  the  archenteron.  In  some  turtles,  on 
the  other  hand,  the  disappearance  of  the  floor  of  the  invagination 
takes  place  at  a  very  early  stage  of  the  infolding,  the  roof  cells 
only  persisting  to  grow  forward  to  form  the  dorsal  wall  of  the  arch- 
enteron. This  interesting  abbreviation  of  the  process  occurring 
in  the  gecko  indicates  the  mode  of  development  which  is  found  in 
the  mammalia. 

The  existence  of  a  prostomial  mesoderm  in  connection  with 
the  primitive  streak  has  already  been  noted,  and  when  the  invagina- 
tion takes  place  it  is  carried  forward  as  a  narrow  band  of  cells  on 


ec 


en 


Fig.  29. — Diagrams  Illustrating   the   Formation  of  the  Gastral  Mesoderm 

IN  the  Gecko. 

e.  Chorda  endoderm;  ec,  ectoderm;  en,  secondary  endoderm;  en,^  primary  endoderm 
gm,  gastral  mesoderm. — i^ill.) 


each  side  of  the  sac  of  secondary  endoderm.  After  the  absorption 
of  the  ventral  wall  of  the  invagination  a  folding  or  turning  in  of  the 
margins  of  the  secondary  endoderm  occurs  (Fig.  29),  whereby  its 
lumen  becomes  reduced  in  size  and  it  passes  off  on  each  side  into  a 
double  plate  of  cells  which  constitute  the  gastral  mesoderm. 
Later  these  plates  separate  from  the  archenteron  as  in  the  lower 
forms.  All  the  prostomial  mesoderm  does  not,  however,  arise 
from  the  primitive  streak  region,  but  a  considerable  amount  also 


56 


FORMATION   OF   THE    GERM   LAYERS 


has  its  origin  from  the  ectoderm  covering  the  yoke  outside  the  lim- 
its of  the  blastoderm  proper,  a  mode  of  origin  which  serves  to  ex- 
plain the  phenomena  later  to  be  described  for  the  mammalia. 


Fig.  30. — Sections  of  Ova  of  a  Bat  Showing  (A)  the  Formation  of  the  Endo- 
DERM  AND  {B  AND  C)  OF    THE  AMNIOTIC    Cavity. — {Van  Betiedeti.) 

In  comparison  with  the  amphibians  and  Amphioxus,  the  rep- 
tilia  present  a  subordination  of  the  process  of  invagination  in  the 
formation  of  the  endoderm,  a  primary  endoderm  making  its  appear- 


FORMATION  OF  THE  GERM  LAYERS  57 

ance  independently  of  an  invagination,  and,  in  association  with 
this  subordination,  there  is  an  early  appearance  of  the  primitive 
streak,  which,  from  analogy  with  what  occurs  in  the  amphibia^ 
may  be  assumed  to  represent  a  portion  of  the  blastopore  which  is 
closed  from  the  very  beginning. 

Turning  now  to  the  mammalia,  it  will  be  found  that  these 
peculiarities  become  still  more  emphasized.  The  inner  cell-mass 
of  these  forms  corresponds  to  the  blastoderm  of  the  reptilian  ovum, 
and  the  first  differentiation  which  appears  in  it  concerns  the 
cells  situated  next  the  cavity  of  the  vesicle,  these  cells  differentiat- 
ing to  form  a  distinct  layer  which  gradually  extends  so  as  to  form  a 
complete  lining  to  the  inner  surface  of  the  enveloping  cells  (Fig.  30, 
A ) .  The  layer  so  formed  is  endodermal  and  corresponds  to  the  pri- 
mary endoderm  of  the  reptiles. 

Before  the  extension  of  the  endoderm  is  completed,  however, 
cavities  begin  to  appear  in  the  cells  constituting  the  remainder  of 
the  inner  mass,  especially  in  those  immediately  beneath  Rauber's 
cells  (Fig.  30,  B),  and  these  cavities  in  time  coalesce  to  form  a 
single  large  cavity  bounded  above  by  cells  of  the  enveloping  layer 
and  below  by  a  thick  plate  of  cells,  the  embryonic  disk  (Fig.  29,  C). 
The  cavity  so  formed  is  the  amniotic  cavity,  whose  further  history 
will  be  considered  in  a  subsequent  chapter. 

It  may  be  stated  that  this  cavity  varies  greatly  in  its  development  in 
different  mammals,  being  entirely  absent  in  the  rabbit  at  this  stage  of 
development  and  reaching  an  excessive  development  in  such  forms  as 
the  rat,  mouse,  and  guinea-pig.  The  condition  here  described  is  that 
which  occurs  in  the  bat  and  the  mole,  and  it  seems  probable,  from 
what  occurs  in  the  youngest  human  embryos  hitherto  observed,  that 
the  processes  in  man  are  closely  similar. 

While  these  changes  have  been  taking  place  a  splitting  of  the 
enveloping  layer  has  occurred  (Fig.  30,  C),  it  becoming  divided 
into  an  outer  layer  whose  cells  unite  to  form  a  syncytium,  and 
an  inner  one  in  which  the  cell  boundaries  remain  distinct.  The 
two  layers  together  form  what  is  termed  the  trophoblast,  from  the 
part  it  subsequently  plays  in  the  nutrition  of  the  embryo,  the 
outer  layer  being  the  plasmodi-trophoblast  and  the  inner  the  cyto- 


58 


FORMATION    OF   THE    GERM   LAYERS 


Irophohlast.  In  the  bat  of  whose  ovum  Fig.  30  C,  represents  a 
section,  that  portion  of  the  cyto-trophoblast  which  forms  the  roof 
of  the  amniotic  cavity  disappears,  only  the  plasmodi-tropho- 
blast  persisting  in  this  region,  but  in  another  form  this  is  not  the 
case,  the  roof  of  the  cavity  bevng  composed  of  both  layers  of  the 
trophoblast. 

A  rabbit's  ovum  in  which  there  is  yet  no  amniotic  cavity  and  no 
splitting  of  the  enveloping  layer  shows,  when  viewed  from  above, 


Pig.  31. — A,  Side  View  of  Ovum  of  Rabbit  Seven  Days  Old  (Kolliker);  B» 
Embryonic  Disk  of  a  Mole  (Heape);  C,  Embryonic  Disk  of  a  Dog's  Ovum  of 
ABOUT  Fifteen  Days  {Bonnet). 

ed.  Embryonic  disk;  hn,  Hensen's  node;  mg,  medullary  groove;  ps,  primitive  streak; 

va,  vascular  area. 


a  relatively  small  dark  area  on  the  surface,  which  is  the  embryonic 
disk.  But  if  it  be  looked  at  from  the  side  (Fig.  31,  A),  it  will  be 
seen  that  the  upper  half  of  the  ovum,  that  half  in  which  the  em- 
bryonic disk  occurs,  is  somewhat  darker  than  the  lower  half,  the 
line  of  separation  of  the  two  shades  corresponding  with  the  edge 
of  the  primary  endoderm  which  has  entended  so  far  in  its  growth 
around  the  inner  surface  of  the  enveloping  layer.  A  little  later  a 
dark  area  appears  at  one  end  of  the  embryonic  disk,  produced  by 


FORMATION    OF    THE    GERM    LAYERS  59 

a  proliferation  of  cells  in  this  region  and  having  a  somewhat  cres- 
centic  form.  As  the  embryonic  disk  increases  in  size  a  longitudinal 
band  makes  its  appearance,  extending  forward  in  the  median  line 
nearly  to  the  center  of  the  disk,  and  represents  the  primitive  streak 
(Fig.  31,  5),  a  slight  groove  along  its  median  line  forming  what  is 
termed  the  primitive  groove.  In  slightly  later  stages  an  especially 
dark  spot  may  be  seen  at  the  front  end  of  the  primitive  streak  and 
is  term^di  Hensen's  node  (Fig.  31,  C,  hn),  while  still  later  a  dark 
streak  may  be  observed  extending  forward  from  this  in  the  median 
line  arid  is  termed  the  head  process  of  the  primitive  streak. 


Fig.  32. — Posterior  Portion  of  a  Longitudinal  Section  through     the  Em- 
bryonic Disk  of  a  Mole. 
hi.    Blastopore,   ec,  ectoderm;   en,  endoderm;   prm,  prostotnial   mesoderm. — {After 

Heape.) 

To  understand  the  meaning  of  these  various  dark  areas  re- 
course must  be  had  to  the  study  of  sections.  A  longitudinal 
section  through  the  embryonic  d^'sk  of  a  mole  ovum  at  the  time 
when  the  crescentic  area  makes  its  appearance  is  shown  in  Fig.  32. 
Here  there  is  to  be  seen  near  the  hinder  edge  of  the  disk  what  is 
potentially  an  opening  (bl) ,  in  front  of  which  the  ectoderm  (ec)  and 
primary  endoderm  (en)  can  be  clearly  distinguished,  while  behind 
it  no  such  distinction  of  the  two  layers  is  visible.  This  stage  may 
be  regarded  as  comparable  to  a  stage  immediately  preceding  the 
invagination  stage  of  the  reptilian  ovum,  and  the  region  behind 
the  blastopore  will  correspond  to  the  reptilian  primitive  streak. 
The  later  forward  extension  of  the  primitive  streak  is  due  to  the 
mode  of  growth  of  the  embryonic  disk.  Between  the  stages  repre- 
sented in  Figs.  32  and  31,  B,  the  disk  has  enlarged  considerably 
and  the  primitive  streak  has  shared  in  its  elongation.  Since  the 
blastopore  of  the  earlier  stage  is  situated  immediately  in  front  of 


6o 


FORMATION    OF    THE    GERM    LAYERS 


the  anterior  extremity  of  the  primitive  streak,  the  point  corres- 
ponding to  it  in  the  older  disk  is  occupied  by  Hensen's  node, 
this  structure,  therefore,  representing  a  proliferation  of  cells 
from  the  region  formerly  occupied  by  the  blastopore. 

As  regards  the  head  process,  it  is  at  first  a  solid  cord  of  cells 


_,i»r 


Pig.  33- — Transverse  Section  of  the  Embryonic  Area  of  a  Dog's   Ovum  at 

ABOUT   THE    StAGE    OF    DEVELOPMENT    SHOWN    IN    FiG,    30,C. 

The  section  passes  through  the  head  process  (Chp);  M,  mesoderm. — (Bonnet.) 

which  grows  forward  in  the  median  line  from  Hensen's  node,  lying 
between  the  ectoderm  and  the  primary  endoderm.  Later  a  lumen 
appears  in  the  center  of  the  cord,  forming  what  has  been  termed 
the  chorda  canal,  and,  in  some  forms,  including  man,  the  canal 


Fig.  34. — Diagram  of  a  Longitudinal  Section  through  the  Embryonic  Disk  of 

A  Mole. 
am,  Amnion;  ce,  chorda  endoderm;  ec,  ectoderm;  nc,  neurenteric  canal;  ps,  primitive 

streak. — (Heape.) 

opens  to  the  surface  at  the  center  of  Hensen's  node.  The  cord 
then  fuses  with  the  subjacent  primary  endoderm  and  then  opens 
out  along  the  line  of  fusion,  becoming  thus  transformed  into  a 
flat  plate  of  cells  continuous  at  either  side  with  the  primary  endo- 
derm (Fig.  ^^,  Chp).     The  portion  of  the  chorda  canal  which 


FORMATION   OF   THE    GERM   LAYERS  6 1 

traverses  Hensen's  node  now  opens  below  into  what  will  be  the 
primitive  digestive  tract  and  is  termed  the  neurenteric  canal  (Fig. 
34,  nc) ;  it  eventually  closes  completely,  being  merely  a  transitory 
structure.  The  similarity  of  the  head  process  to  the  invagination 
which  in  the  reptilia  forms  the  secondary  endoderm  seems  clear, 
the  only  essential  difference  being  that  in  the  mammalia  the  head 
process  arises  as  a  solid  cord  which  subsequently  becomes  hollow, 
instead  of  as  an  actual  invagination.  The  difference  accounts 
for  the  occurrence  of  Hensen's  node  and  also  for  the  mode  of  forma- 
tion of  the  neurenteric  canal,  and  cannot  be  considered  as  of  great 
moment  since  the  development  of  what  are  eventually  tubular 


Fig,  35. — Transverse   Section  through  the  Embryonic  Disk  of  a  Rabbit. 
ch.  Chorda  endoderm;  ee,  ectoderm;  en,  endoderm;  gm,  gastral  mesoderm. — {After 

van  Beneden.) 

structures  {e.g.,  glands)  as  solid  cords  of  cells  which  subsequently 
hollow  out  is  of  common  occurrence  in  the  mammalia.  It  should 
be  stated  that  in  some  mammals  apparently  the  most  anterior 
portion  of  the  roof  of  the  archenteron  is  formed  directly  from  the 
cells  of  the  primary  endoderm,  which  in  this  region  are  not  re- 
placed by  the  head  process,  but  aggregate  to  form  a  compact  plate 
of  cells  with  which  the  anterior  extremity  of  the  head  process 
unites.  Such  a  condition  would  represent  a  further  modification 
of  the  original  condition. 

As  regards  the  formation  of  the  embryonic  mesoderm  it  is 
not  always  possible  to  recognize  both  the  prostomial  and 
gastral  mesoderm  in  the  mammalian  ovum.  A  mass  of  pros- 
tomial mesoderm  is  formed  from  the  primitive  streak  and  as  the 
head  process  grows  forward  a  band  of  this  mesoderm  extends  for- 
ward on  either  side  of  it,  but  whether  contributions  are  added  to 


62 


FORMATION    OF    THE    GERM   LAYERS 


these  bands  from  the  head  process  is  uncertain.  Later  on  the 
medial  margins  of  the  bands  come  into  intimate  relation  with  the 
head  process  or  chorda  endoderm,  just  where  this  unites  with  the 
primitive  endoderm,  and  an  appearance  may  be  presented  closely 
similar  to  that  shown  in  reptilia  (compare  Fig.  29,  D  and  Fig.  35). 
If,  in  the  mammalia  the  head  process  tissue  takes  no  part  in  the 
formation  of  these  lateral  plates  of  mesoderm  it  may  be  supposed 
that  a  concentration  of  the  development  has  taken  place,  the 
ABC  D 


Pig.  36. — Diagrams  Illustrating  the  Relations  of  the  Chick  Embryo  to  the 
Primitive  Streak  at  Different  Stages  of  Development, — {Peebles.) 

head  process  being  composed  purely  of  chorda  endoderm,  while 
the  mesoderm  associated  with  this  in  the  reptilia  now  takes  its 
origin  directly  from  the  primitive  streak.  The  lateral  plates  of 
mesoderm  are  at  first  solid  (Fig.  35,  gm),  but  their  cells  early  ar- 
range themselves  in  two  layers,  between  which  a  space,  termed 
the  body-cavity  or  coelom,  later  appears. 

In  addition  to  this  embryonic  mesoderm  a  certain  amount, 
sometimes  quite  large,  of  the  same  layer  is  found  lining  the  inner 
surface  of  the  cytotrophoblast,  lying  between  this  and  the  primary 


SIGNIFICANCE    OF   THE    GERM   LAYERS  63 

endoderm.  The  exact  source  of  this  extra-embryonic  mesoderm 
is  uncertain,  though  it  seems  probable  that  it  is  formed  in  situ, 
and  is  perhaps  represented  in  the  reptilian  ovum  by  the  cells  which 
underlie  the  ectoderm  in  the  regions  peripheral  to  the  blastoderm 
proper  (seepage  55). 

It  has  been  experimentally  determined  (Assheton,  Peebles)  that  in 
the  chick,  whose  embryonic  disk  presents  many  features  similar  to  those 
of  the  mammalian  ovum,  the  central  point  of  the  unincubated  disk 
corresponds  to  the  anterior  end  of  the  primitive  streak  and  to  the 
point  situated  immediately  behind  the  heart  of  the  later  embryo  and 
immediately  in  front  of  the  first  mesodermic  somite  (see  p.  79),  as 
shown  in  Fig.  36.  If  these  results  be  regarded  as  applicable  to  the 
human  embryo,  then  it  may  be  supposed  that  in  this  the  head  region 
is  developed  from  the  portion  of  the  embryonic  disk  situated  in  front 
of  Hensen's  node,  while  the  entire  trunk  is  a  product  of  the  region 
occupied  by  the  node. 

The  Significance  of  the  Germ  Layers. — The  formation  of  the 
three  germ  layers  is  a  process  of  fundamental  importance,  since 
it  is  a  differentiation  of  the  cell  units  of  the  ovum  into  tissues  which 
have  definite  tasks  to  fulfil.  As  has  been  seen,  the  first  stage  in 
the  development  of  the  layers  is  the  formation  of  the  ectoderm  and 
endoderm,  or,  if  the  physiological  nature  of  the  layers  be  considered, 
it  is  the  differentiation  of  a  layer,  the  endoderm,  which  has  princi- 
pally nutritive  functions.  In  certain  of  the  lower  invertebrates, 
the  class  Coelentera,  the  differentiation  does  not  proceed  beyond 
this  diploblastic  stage,  but  in  all  higher  forms  the  intermediate 
layer  is  also  developed,  and  with  its  appearance  a  further  division 
of  the  functions  of  the  organism  supervenes,  the  ectoderm,  situated 
upon  the  outside  of  the  body,  assuming  the  relational  functions, 
the  endoderm  becoming  still  more  exclusively  nutritive,  while  the 
remaining  functions,  supportive,  excretory,  locomotor,  reproduc- 
tive, etc.  are  assumed  by  the  mesoderm. 

The  manifold  adaptations  of  development  obscure  in  certain 
cases  the  fundamental  relations  of  the  three  layers,  certain  por- 
tions of  the  mesoderm,  for  instance,  failing  to  differentiate  simul- 
taneously with  the  rest  of  the  layer  and  appearing  therefore  to  be 
a  portion  of  either  the  ectoderm  or  endoderm.     But,  as  a  rule,  the 


64  SIGNIFICANCE    OF   THE    GERM   LAYERS 

layers  are  structural  units  of  a  higher  order  than  the  cells,  and  since 
each  assumes  definite  physiological  functions,  definite  structures 
have  their  origin  from  each. 

Thus  from  the  ectoderm  there  develop: 

1.  The  epidermis  and  its  appendages,  hairs,  nails,  epidermal 
glands,  and  the  enamel  of  the  teeth. 

2.  The  epithelium  lining  the  mouth  and  the  nasal  cavities,  as 
well  as  that  lining  the  lower  part  of  the  rectum. 

3.  The  nervous  system  and  the  nervous  elements  of  the  sense- 
organs,  together  with  the  lens  of  the  eye. 

From  the  endoderm  develop: 

1.  The  epithelium  lining  the  digestive  tract  in  general,  together 
with  that  of  the  various  glands  associated  with  it,  such  as  the  liver 
and  pancreas. 

2.  The  lining  epithelium  of   the   larynx,  trachea,  and  lungs. 

3.  The  epithelium  of  the  bladder  and  urethra  (in  part). 
From  the  mesoderm  there  are  formed : 

1.  The  various  connective  tissues,  including  bone  and  the  teeth 
(except  the  enamel) . 

2.  The  muscles,  both  striated  and  non-striated. 

3.  The  circulatory  system,  including  the  blood  itself  and  the 
lymphatic  system. 

4.  The  lining  membrane  of  the  serous  cavities  of  the  body. 

5.  The  kidneys  and  ureters. 

6.  The  internal  organs  of  reproduction. 

From  this  list  it  will  be  seen  that  the  products  of  the  mesoderm 
are  more  varied  than  those  of  either  of  the  other  layers.  Among 
its  products  are  organs  in  which  in  either  the  embryonic  or  adult 
condition  the  cells  are  arranged  in  a  definite  layer,  while  in  other 
structures  its  cells  are  scattered  in  a  matrix  of  non-cellular  ma- 
terial, as,  for  example,  in  the  connective  tissue,  bone,  cartilage,  and 
the  blood  and  lymph.  It  has  been  proposed  to  distinguish  these 
two  forms  of  mesoderm  as  mesothelium  and  mesenchyme  respectively , 
a  distinction  which  is  undoubtedly  convenient,  though  probably 
devoid  of  the  fundamental  importance  which  has  been  attributed 
to  it  by  some  embryologists. 


LITERATURE  65 


LITERATURE 

R.  Assheton:    "The  Reinvestigation  into  the  Early  Stages  of  the  Development  of 

the  Rabbit,"  Quarterly  Journ.  of  Microsc.  Science,  xxxvii,  1894.  - 

R.  Assheton:  "The  Development  of  the  Pig  During  the  First  Ten  Days,"  Qimrterly 

Journ.  of  Micros.  Science,  xli,  1898. 
R.  Assheton:  "The  Segmentation  of  the  Ovum  of  the  Sheep,  with  Observations  on 

the  Hypothesis  of  a  Hypoblastic  Origin  for  the  Trophoblast,"  Quarterly  Journ. 

of  Microsc.  Science,  xli,  1898. 
E.VAN  Beneden:  "Recherches  sur  I'embryologie  des  Mammiferes.    De  la  segmenta- 
tion, de  la  formation  de  la  cavit6  blastodermique  et  de  I'embryon  didermique 

chez  le  Murin,"  Arch,  de  Biol.,  xxvi,  191 1. 
E.  VAN  Beneden:  "Recherches  sur  I'embryologie  des  Mammiferes  II:  De  la  ligne 

primitive,  du  prolongement  cephalique,  de  la  notochorde  et  du  mesoblaste  chez 

le  lapin  et  chez  le  murin,"  Arch,  de  Biol.,  xxvii,  1912. 
R.  Bonnet:  "Beitrage  zur  Embryologie  der  Wiederkauer  gewonnen  am  Schafei," 

Archiv  fur  Anat.  und  Physiol.,  Anat.  Ahth.,  1884  and  1889., 
R.  Bonnet:  "Beitrage  zur  Embryologie  des  Hundes,"  Anat.  Hefte,  ix,  1897. 
G.  Born:  "Erste  Entwickelungsvorgange,"  Ergehnisse  der  Anat.  und  Entwicklungs- 

gesch.,  I,  1892. 

E.  G.  Conklin:  "The  Cause  of  Inverse  Symmetry,"  Anatom.  Anzeiger,  xxiii,  1903. 

A.  C.  Eycleshymer:  "The  Early  Development  of  Amblystoma  with  Observations 

on  Some  Other  Vertebrates,"  Jour,  of  Morphol.,  x,  1895. 

B.  Hatschek:  "Studien  iiber  Entwicklung  des  Amphioxus,"  Arbeiten  aus  dem  zoolog. 

Instit.  zu  Wien,  iv,  1881. 

W.  Heape:  "The  Development  of  the  Mole  (Talpa  europaea),"  Quarterly  Journ.  of 
Micros.  Science,  xxiii,  1883. 

G.  C.  Huber:  "On  the  Anlage  and  Morphogenesis  of  the  Chorda  dorsalis  in  Mam- 
malia, in  particular  the  Guinea-pig  (Cavia  Cobaya)".     Anat.  Record  xiv,  1918. 

A.  A.  W.  Hubrecht:  "Studies  on  Mammalian  Embryology  II:  The  Development  of 
the  Germinal  Layers  of  Sorex  vulgaris,"  Quarterly  Journ.  of  Microsc.  Science, 
XXXI,  1890. 

F.  Keibel:  "Studien   zur   Entwicklungsgeschichte   des    Schweines,"    Morpholog. 

Arbeiten,  iii,  1893. 
F.  Keibel:  "Die  Gastrulation  und  die  Keimblattbildung  der  Wirbeltiere,"  Ergeb- 

nisse  der  Anat.  und  Entwicklungsgesch.,  x,  1901. 
M.  KuNSEMtJLLER :  "Die  Eifurchung  des  Igels  (Erinaceus  europaeus  L.),"  Zeitschr. 

fUr  wissensch.  Zool.,  lxxxv,  1906. 
K.  MiTSUKURi  and  C.  Ishikawa:  "On  the  Formation  of  the  Germinal  Layers  in 

Chelonia,"  Quarterly  Journ.  of  Microsc.  Science,  xxvii,  1887. 
F.  Peebles:  "The  Location  of  the  Chick  Embryo  upon  the  Blastoderm,"  Journ.  of 

Exper.  Zool.,  i,  1904. 
E.  Selenka:  "Studien  iiber  Entwickelungsgeschichte  der  Thiere,"  4tes  Heft,  1886- 

87;  5tes  Heft,  1891-91. 
J.  Sobotta:  "Die  Befruchtung  und  Furchung  des  Eies  der  Maus,"  Archiv  fiir  mik- 

rosk.  Anat.,  xlv,  1895. 
5 


66  LIIERATURE 

J.  Sobotta:  "Die  Furchung  des  Wirbelthiereies,"  Ergehnisse  der  Anat.  und  Entwicke- 

hingsgeschichte,  vi,  1897. 
J.  Sobotta:  "Neuere  Anschauungen  Uber  Entstehung  der  Doppel   (miss)  bild- 

ungen,  mit  besonderer  Beriicksichtigung  der  menschlichen  Zwillingsgeburten," 

Wiirzhurger  Abhandl.,  i,  1901. 
H.  H.  Wilder:  "Duplicate  Twins  and  Double  Monsters,"  Amer.  Jour,  of  Anat.,  iii, 

1904- 
L.  Will:  "Beitrage  zur  Entwicklungsgeschichte  der  Reptilien,"  Zoolog.  JahrhUcher 
A  hth.  fur  A  nat. ,  vi,  1 893 . 


CHAPTER  III 

THE  MEDULLARY  GROOVE,  NOTOCHORD,  AND  MESO- 
DERMIC  SOMITES 

In  the  preceding  chapter  the  development  of  the  mammalian 
ovum  has  been  described  up  to  and  including  the  formation  of  the 
three  germinal  layers.  The  earlier  stages  of  development  there 
described  are  practically  unknown  in  the  human  ovum,  but  for  the 
stages  subsequent  to  the  establishment  of  the  germinal  layers 
human  material  is  available,  and  it  will,  therefore,  now  be  con- 
venient to  consider  the  structure  of  the  younger  human  ova  at 
present  known  and  to  trace  in  them  the  appearance  and  develop- 
ment of  such  structures  as  the  primitive  streak,  the  head  process 
and  the  gastral  mesoderm. 

The  youngest  human  ovum  at  present  known  is  that  described 
by  Bryce  and  Teacher,  but,  unfortunately,  it  presents  certain 
features  that  are  evidently  abnormal,  so  that  it  becomes  doubtful 
how  far  it  may  be  accepted  as  representing  the  typical  condition. 
The  trophoblast,  which  was  very  thick  and  clearly  differentiated 
into  two  layers,  enclosed  a  space  whose  diameter  was  about  0.63 
mm.  and  which  was  largely  occupied  by  a  loose  syncytial  tissue. 
Toward  the  center  of  this  was  an  irregular  cavity  in  which  were  two 
vesicles,  quite  separate  from  one  another  and  probably  together 
representing  the  embryo,  the  smaller  one  being  the  amniotic  cavity 
and  the  larger  one  the  cavity  lined  by  the  endoderm  and  known  as 
the  yolk-sac  (Fig.  37).  The  separation  of  these  two  structures  is 
apparently  an  abnormality  and  it  is  possible  that  the  cavity  in 
which  they  lie  is,  as  Bryce  and  Teacher  suggest,  an  artefact  pro- 
duced by  contraction  of  the  syncytial  tissue  during  the  preserva- 
tion of  the  ovum. 

If  comparison  of  this  ovum  with  those  of  other  mammals  is 
warranted,  it  may  be  likened  to  that  of  the  bat  as  shown  in  Fig. 

67 


68 


THE    MEDULLARY   GROOVE 


30,  C,  with  the  difference  that  the  space  between  the  primary  endo- 
derm  and  the  trophoblast  is  greatly  enlarged  in  the  human  ovum 
and  is  occupied  by  loose  syncytial  tissue,  which  may  be  termed  the 
cellular  magma.  This  condition  may  be  represented  diagrammat- 
ically  as  in  Fig.  39,  A,  in  which  the  magma  is  represented  as  some- 
what condensed  around  the  amniotic  cavity  and  yolk-sac  and  upon 
the  inner  surface  of  the  trophoblast.     Whence  this  magma  tissue 


Fig.  37.- 


-From  a  Reconstruction  of  the  Bryce-Teacher  Ovum. 
^Bryce-Teacher.) 


is  derived  is  as  yet  uncertain,  but  it  seems  probable  that  it  repre- 
sents a  precocious  development  of  the  extra-embryonic  mesoblast, 
i.e.,  of  that  portion  of  the  mesoblast  that  lies  outside  the  actual 
limits  of  the  embryonic  rudiment  (see  page  62). 

Somewhat  older  are  the  ova  described  by  Peters,  Fetzer,  Jung, 
Linzenmeier  and  Herzog.  The  Peters  ovum  was  taken  from  the 
uterus  of  a  woman  who  had  committed  suicide  one  calendar 
month  after  the  last  menstruation,  and  it  measured  about  i  mm. 
in  diameter.  The  entire  inner  surface  of  the  trophoblast  (Fig.  $S  ce) 
was  lined  by  a  layer  of  mesoderm  {cm),  which,  on  the  surface 
furthest  away  from  the  uterine  cavity,  was  considerably  thicker 


THE    MEDULLARY    GROOVE  69 

than  elsewhere,  forming  an  area  of  attachment  of  the  embryo  to 
the  wall  of  the  ovum.  In  the  substance  of  this  thickening  was  the 
amniotic  cavity  (am),  whose  roof  was  formed  by  flattened  cells,- 
which,  at  the  sides,  became  continuous  with  a  layer  of  columnar 
cells  forming  the  floor  of  the  cavity  and  constituting  the  embryonic 
ectoderm  (ec) .  Immediately  below  this  was  a  layer  of  mesoderm 
(m)  which  split  at  the  edge  of  the  embryonic  disk  into  two  layers, 
one  of  which  became  continuous  with  the  mesodermic  thickening 


^      am^ 


cm^ 


..   .  ....   ^ 


en 


J. 


Fig    38. — Section  of  Embryo  and  Adjacent  Portion  of  an  Ovum  of  i  mm. 
am.  Amniotic  cavity;  ce,  chorionic  ectoderm;  cm,  chorionic  mesoderm;  ec,  embryonic 
ectoderm;  en,  endoderm;  m,  embryonic  mesoderm;  ys,  yolk-sac. — (Peters.) 

and  so  with  the  layer  of  mesoderm  lining  the  interior  of  the  tropho- 
blast,  while  the  other  enclosed  a  sac  lined  by  a  layer  of  endodermal 
cells  and  forming  the  yolk-sac  (ys).  The  total  length  of  the 
embryo  was  0.19  mm.,  and  so  far  as  its  ectoderm  and  mesoderm  are 
concerned  it  might  be  described  as  a  flat  disk  resting  on  the  surface 
of  the  yolk-sac,  though  it  must  be  understood  that  the  yolk-sac 
also  to  a  certain  extent  forms  part  of  the  embryo. 

This  embryo  seems  to  be  in  an  early  stage  of  the  primitive 
streak  formation,  before  the  development  of  the  head  process.     On 


70 


THE    MEDULLARY    GROOVE 


comparing  it  with  the  stage  of  development  represented  in  Fig. 
39,  A,  it  will  be  seen  to  present  some  important  advances.  The 
cavity  (Fig.  39,  B,  C,)  into  which  the  yolk-sac  projects  is  unrepre- 
sented in  Fig.  39,  A,  and  seems  to  have  been  formed  by  the  concen- 
tration of  the  cells  of  the  cellular  magma  upon  the  trophoblast 
and  around  the  yolk-sac  and  amniotic  cavity.  The  cavity  is  oc- 
cupied by  a  mucous  fluid,  destitute  of  cellular  elements  at  this 
stage  and  forming  what  is  termed  the  reticular  magma,  and  the  size 
of  the  human  ovum  at  this  stage  and  later  is  mainly  due  to  the 
rapid  growth  of  this  cavity.     The  fact  that  the  cavity  is  every- 


^me 


Fig.  39. — Diagrams  to  show  the  Probable  Relationships  of  the  Parts  in  the 

Embryos  Represented  in  Pigs.  37  and  38 
ac.  Amniotic  cavity;  c,  extra-embryonic  ccelom;  Co,  embryonic  ccelom;  Cy,  cyto-tro- 
phoblast;   m,  cellular  magma;   me,  chorionic  mesoderm;  PI,  plasmodi-trophoblast; 
y,  yolk  sac. 

where  bounded  by  mesoderm  suggests  that  it  is  the  extra-em- 
bryonic body-cavity,  formed  precociously  before  the  splitting  of 
the  embryonic  mesoderm  (see  p.  62).  It  seems  more  probable, 
however,  that  the  extra-embryonic  ccelom  is  really  represented  by 
certain  cavities  lined  with  a  flattened  epithelium  which  occur  in  the 
immediate  neighborhood  of  the  embryo  (Figs.  38  and  39,  B,  Co). 
These,  in  later  stages,  probably  become  continuous  with  the 
cavity  occupied  by  the  reticular  magma  by  the  breaking  down  of 
the  separating  walls,  and  if  this  be  the  correct  interpretation  of  the 


THE   MEDULLARY   GROOVE 


71 


facts  the  extra-embryonic  coelom  is  formed  precociously  in  the 
human  ovum  and  the  cavity  occupied  by  the  reticular  magma 
eventually  becomes  part  of  it.     From  this  stage  onward  the  tro— 
phoblast  and  the  layer  of  mesoderm  lining  it  may  together  be 


z-^;. 


%. 


Fig.  40. — The  Embryo  v.  H.  of  von  Spee.     The  Left  Half  of  the  Chorion  has 

BEEN  Removed  to  show  the  Embryo. 

a.  Amniotic  cavity;  h,  belly-stalk;  ch,  chorion;  d,  yolk-sac;  e,  extra-embryonic  coelom; 

k,  embryonic  disk;  2,  chorionic  villus. — {von  Spee.) 

spoken  of  as  the  chorion,  the  mesoderm  layer  being  termed  the 
chorionic  mesoderm. 

A  little  older  again  than  the  Peters  and  Herzog  ova  are  those 
described  by  Strahl  and  Beneke,  and  by  von  Spee  (embryo  v.  H.), 
the  chorionic  cavity  of  the  former  two  having  an  average  diameter 


Fig.  41. — Embryo  from  the  Benkkk  Ovum,  the  Roof  of  the  Amniotic  Cavity 

HAVING  been  Removed. 
From  a  model,     h,   Belly-stalk;   p.g.,   primitive  groove;  y,   yolk-sac. — (Slrahl  and 

Beneke.) 


of  about  2.4  mm.,  while  the  corresponding  size  of  the  latter  two  was 
somewhat  less  than  4.0  mm.  Notwithstanding  the  considerable 
increase  in  the  size  of  these  older  ova,  due  to  the  continued  increase 
in  the  size  of  the  extra-embryonic  coelom,  the  embryos  are  but 


72  THE   MEDULLARY   GROOVE 

little  advanced  beyond  the  stage  shown  by  the  Peters  embryo. 
The  thickening  of  the  chorionic  mesoderm  that  encloses  the  amni- 
otic cavity  has  now  become  smaller  relatively  to  the  extent  of  the 
chorion  and  forms  a  pedicle,  known  as  the  belly-stalk  (Fig.  40,  h. 
at  the  extremity  of  which  is  the  yolk-sac  {d).  Furthermore,  the 
amniotic  cavity  (a)  now  lies  somewhat  eccentrically  in  this  pedicle, 
being  near  what  may  be  termed  its  anterior  surface,  and  the  entire 
embryo  projects  like  a  papilla  from  the  inner  surface  of  the  chorion 
into  the  extra-embryonic  coelom.  Fig.  41  is  from  a  model  of  the 
Beneke  embryo,  detached  from  the  chorion  by  cutting  through  the 
belly-stalk,  and  with  the  roof  of  the  amniotic  cavity  removed. 
The  embryonic  disk,  thus  exposed,  is  an  oval  plate,  resting,  as  it 
were,  on  the  yolk-sac,  and  quite  smooth  except  for  a  slight  longi- 


FiG.  42. — Embryo  from  the  Frassi  Ovum,  the  Roof  of  the  Amniotic  Cavity 

HAVING  been  Removed. 
From  a  model.     &,  belly-stalk;  p.g.,  primitive  groove;  mg,  medullary  groove;  «. 

neurenteric  canal. — (Frassi.) 

tudinal  groove  upon  its  posterior  portion.  This  is  the  primitive 
groove  and  sections  passing  through  it  show  the  primitive  streaky 
consisting  of  a  sheet  of  mesoderm  interposed  between  the  ectoderm 
and  endoderm,  as  in  the  Peters  embryo,  and  but  poorly  defined 
from  the  other  two  layers.  From  its  anterior  edge  a  median  proc- 
ess extends  forward  for  a  short  distance  and  is  the  head  process 
(see  p.  60).  In  front  and  to  the  sides  of  this  there  is  as  yet  no 
mesoderm  intervening  between  the  ectoderm  and  endoderm. 

The  embryonic  disk  of  the  Beneke  embryo  measured  0.75  mm. 
in  length.  That  of  an  embryo  described  by  Frassi  (Fig.  42)  was 
i.iy^mm.  in  length,  and  in  correspondence  with  its  greater  size,  it 
presents  some  advances  in  structure  that  are  of  interest.     As  in 


THE    MEDULLARY    GROOVE  73 

the  younger  embryo  one  sees  a  distinct  primitive  groove  on  the 
posterior  portion  of  the  embryonic  disk,  but  the  groove  terminates 
anteriorly  at  a  distinct  pore  (n),  which  perforates  the  disk  and 
opens  ventrally  into  the  yolk-sac.  This  is  the  neurenteric  canal 
(see  p.  6i)  and  in  front  of  it  a  groove  extends  forward  in  the  me- 
dian line  almost  to  the  anterior  edge  of  the  embryonic  disk  and  is 
evidently  the  first  indication  of  the  medullary  groove,  whose  walls 
are  destined  to  give  rise  to  the  central  nervous  system.  Sections 
passing  through  the  region  of  the  medullary  groove  show,  lying 


am 


Fig.  43. — Section  through  the  Prassi  Embryo  just  in  Front  of  the  Neuren- 
teric Canal. 
am.  Amniotic  cavity;  gm,  gastral  mesoderm;  hp,  head  process;  mp,  medullary  plate; 

ys,  yolk-sac. — (Frassi.) 

beneath  it,  the  head  process  (Fig.  43,  hp),  already  fused  with  the 
endoderm  (compare  p.  61),  and  on  each  side  of  the  process  is  a 
plate  of  mesoderm  (gm),  representing  the  gastral  mesoderm  of 
lower  forms  (see  Figs.  29  and  35),  but  not  as  yet  showing  any 
indications  of  splitting  into  the  two  layers  that  bound  the  embry- 
onic coelom  (see  p.  62). 

This  is  just  beginning  to  appear  in  an  embryo,  also  described 
by  von  Spee  and  known  as  embryo  Gle.  It  measured  1.54  mm.  in 
length  and  is  closely  similar,  in  general  appearance,  to  an  embryo 
described  by  Eternod  and  measuring  1.34  mm.  in  length  (Fig.  44). 
It  differs  from  the  Frassi  embryo  most  markedly  in  that  the  poste- 
rior streak  region,  is  bent  ventrally  so  as  to  lie  almost  at  a  right 
angle  with  the  anterior  portion.  As  a  result  the  belly-stalk  arises 
from   the   ventral   surface   of   the   embryo  instead   of   from   its 


74 


THE    MEDULLARY    GROOVE 


Fig.  44. — Embryo  1.34  mm.  Long. 
al,  AUantois;  am,  amnion;  bs,  belly-stalk;  h,  heart;  m,  medullary  groove;  nc, 
neurenteric  canal;  pc,  caudal  protuberance;  ps,  primitive  streak;  ys,  yolk-stalk. — 
{Eternod.) 


THE    MEDULLARY    FOLDS  75 

posterior  extremity,  near  which  the  opening  of  the  neurenteric 
canal  (Fig.  43,  nc)  is  now  situated,  almost  the  whole  length  of  the 
surface  seen  in  dorsal  view  being  occupied  by  the  medullary  groove 
(w),  which,  in  the  embryo  G/e,  is  bounded  laterally  by  distincT 
ridges,  the  medullary  folds. 

In  the  Kromer  embryo  Klh  (Fig.  45),  measuring  1.8  mm.  in 
length,  a  new  feature  has  made  its  appearance.  The  medullary 
folds  have  become  quite  high,  and  lateral  to  them  there  is  on  each 
side  a  series  of  five  or  six  oblong  elevations,  which  represent  what 
are  termed  mesodermic  somites  and  are  due  to  divisions  of  the 
underlying  mesoderm. 


Fig.  45. — Model  of  the  Kromer  Embryo  Klh  seen  from  the  Dorsal  Surface, 
THE  Roof  of  the  Amniotic  Cavity  having  been  Removed. — {Keibel  and  Elze.) 

Instead  of  proceeding  with  a  description  of  the  external  form  of 
still  older  embryos  it  will  be  convenient  to  consider  the  further 
development  of  certain  structures  whose  appearance  has  already 
been  noted,  namely,  the  head  process,  the  medullary  folds  and  the 
mesodermic  somites,  and  first  of  all  the  medullary  folds  may  be 
considered. 

The  Medullary  Folds. — The  two  folds  are  continuous  ante- 
riorly, but  behind  they  are  at  first  separate,  the  anterior  portion  of 
the  primitive  streak  lying  between  them.  In  forms,  such  as  the 
Reptilia,  which  possess  a  distinct  blastopore,  this  opening  lies  in 
the  interval  between  the  two,  and  consequently  is  in  the  floor  of  the 
medullary  groove,  and  in  the  mammalia,  even  though  no  well-de- 
fined blastopore  is  formed,  yet  at  the  time  of  the  formation  of  the 


76 


THE    MEDULLARY    FOLDS 


medullary  fold  an  opening  breaks  through  at  the  anterior  end  of 
the  primitive  streak  in  the  region  of  Hensen's  node,  and  places  the 
cavity  lying  below  the  endoderm  in  communication  with  the  space 
bounded  by  the  medullary  folds.  The  canal  so  formed  is  termed 
the  neur enteric  canal  (Figs.  44  and  46,  nc)  and  is  so  called  because 
it  unites  what  will  later  become  the  central  canal  of  the  nervous 


Pig.  46. — Diagram  of  a  Longitudinal  Section  through  the  Embryo  Gle,  Meas- 
uring 1.54  MM.  IN  Length. 

al,  Allantois;  am,  amnion;  B,  belly-stalk;  ch,  chorion;  h,  heart;  nc,  neurenteric  canal; 
V,  chorionic  villi;  Y,  yolk-sac. — {von  Spee.) 

system  with  the  intestine  (enteron) .  The  significance  of  this  canal 
has  already  been  discussed  (p.  61);  it  is  of  very  brief  persistence, 
closing  at  an  early  stage  of  development  so  as  to  leave  no  trace  of 
its  existence. 

As  development  proceeds  the  medullary  folds  increase  in  height 
and  at  the  same  time  incline  toward  one  another  (Fig.  45),  so  that 
their  edges  finally  come  into  contact  and  later  fuse,  the  two  ecto- 


THE   NOTOCHORD  77 

dermal  layers  forming  the  one  uniting  with  the  corresponding 
layers  of  the  other  (Fig.  47) .  By  this  process  the  medullary  groove 
becomes  converted  into  a  medullary  canal  which  later  becomes  the 
central  canal  of  the  spinal  cord  and  the  ventricles  of  the  brain,  the 
ectodermal  walls  of  the  canal  thickening  to  give  rise  to  the  central 
nervous  system.  The  closure  of  the  groove  does  not,  however, 
take  place  simultaneously  along  its  entire  length,  but  begins  in 
what  corresponds  to  the  neck  region  of  the  adult  and  thence  pro- 
ceeds both  anteriorly  and  posteriorly,  the  extension  of  the  fusion 


Fig.  47. — Diagrams  showing  the  Manner  ov  ihe  Closure  of  the  Medullary 

Groove. 

taking  place  rather  slowly,  however,  especially  anteriorly,  so 
that  an  anterior  opening  into  the  otherwise  closed  canal  can  be 
distinguished  for  a  considerable  period  (Fig.  54). 

The  Notochord. — While  these  changes  have  been  taking  place 
in  the  ectoderm  of  the  median  line  of  the  embryonic  disk,  modifica- 
tions of  the  subjacent  endoderm  have  also  occurred.  This  endo- 
derm,  it  will  be  remembered,  was  formed  by  the  head  process  of  the 
primitive  streak,  and  was  a  plate  of  cells  continuous  at  the  sides 
with  the  pimary  endoderm  and  extending  forward  as  far  as  what 
will  eventually  be  the  anterior  part  of  the  pharynx.     Along  the 


78 


THE   NOTOCHORD 


line  of  its  junctioD  with  the  primary  endoderm  it  is  in  relation  to 
the  medial  edges  of  the  lateral  plates  of  mesoderm,  which  are 
comparable  to  the  gastral  mesoderm  of  lower  forms,  and  it  itself 
produces  an  important  embryonic  organ  known  as  the  notochord 
or  chorda  dorsdis,  whence  the  term  chorda  endoderm  sometimes 
applied  to  it. 

I  After  it  has  united  with  the  primary  endoderm  the  chorda  en- 
doderm is  a  flat  band,  but  later  it  becomes  somewhat  curved, 
concave  towards  the  yolk-sack  (Fig.  48,  A),  and,  the  curvature 


Fig.  48. — Transverse  Sections  through  Mole  Embryos  showing  the  Forma- 
tion OF  the  Notochord. 
ec.  Ectoderm;  en,  endoderm;  m,  mesoderm;  nc,  notochord. — (Heape.) 


increasing,  the  edges  of  the  plate  come  into  contact  and  finally 
fuse  (Fig.  48,  B),  the  edges  of  the  primary  endoderm  at  the  same 
time  uniting  beneath  the  chordal  tube  so  formed,  so  that  this 
layer  becomes  a  continuous  sheet,  as  it  was  at  its  first  appearance. 
A  distinct  lumen,  the  secondary  chordal  canal,  may  occur  in  the 
the  chordal  tube,  but  it  is  soon  obliterated  by  the  enlargement  of 
cells  which  bound  it,  and  these  cells  later  undergo  a  peculiar  trans- 
formation whereby  the  chordal  tube  is  converted  into  a  solid 
elastic  rod  surrounded  by  a  cuticular  sheath  secreted  by  the  cells. 
The  notochord  lies  at  first  immediately  beneath  the  median  line  of 
the  medullary  groove,  between  the  ectoderm  and  the  endoderm, 
and  has  on  either  side  of  it  the  mesodermal  plates.     It  does  not. 


THE    MESODERMIC    SOMITES  79 

however,  quite  reach  the  anterior  extremity  of  the  head,  but 
terminates  beneath  the  cerebral  portion  of  the  medullary  canal  at  a 
point  just  caudad  to  where  the  hypophysis  will  be  developed.  It 
is  a  temporary  structure  of  which  only  rudiments  persist  in  the~ 
adult  condition  in  man,  but  it  is  a  structure  characteristic  of  all 
vertebrate  embryos  and  persists  to  a  more  or  less  perfect  extent 
in  many  of  the  fishes,  being  indeed  the  only  axial  skeleton  possessed 
by  Amphioxus.  In  the  higher  vertebrates  it  is  almost  completely 
replaced  by  the  vertebral  column,  which  develops  around  it  in  a 
manner  to  be  described  later. 

The  Mesodermic  Somites. — Turning  now  to  the  middle 
germinal  layer,  it  will  be  found  that  in  it  also  important  changes 
take  place  during  the  early  stages  of  development.  The  probable 
mode  of  development  of  the  extra-embryonic  mesoderm  and  body 
cavity  has  already  been  described  (p.  70)  and  attention  may  now 
be  directed  toward  what  occurs  in  the  embryonic  mesoderm.  In 
both  the  Peters  embryo  and  the  embryo  v.  H.  described  by  vonSpee 
this  portion  of  the  mesoderm  is  represented  by  a  plate  of  cells  lying 
between  the  ectoderm  and  endoderm  and  continuous  at  the  edges 
of  the  embryonic  area  both  with  the  layer  of  extra-embryonic  meso- 
derm which  surrounds  the  yolk-sac  and,  through  the  mesoderm  of 
the  belly-stalk,  with  the  chorionic  mesoderm  (Fig.  38).  In  older 
embryos,  such  as  the  embryo  Gle  of  Graf  Spee  and  the  younger 
embryo  described  by  Eternod  (Fig.  44),  the  mesoderm  no  longer 
forms  a  continuous  sheet  extending  completely  across  the  em- 
bryonic disk,  but  is  divided  into  two  lateral  plates,  in  the  interval 
between  which  the  ectoderm  of  the  floor  of  the  medullary  groove 
and  the  chorda  endoderm  are  in  close  contact  (Fig.  49) .  The 
cha^iges  which  next  occur  have  not  as  yet  been  observed  in  the 
human  embryo,  though  they  probably  resemble  those  described 
in  other  mammalian  embryos,  and  the  phenomena  which  occur  in 
the  sheep  may  serve  to  illustrate  their  probable  nature. 

It  has  been  seen  that  in  the  stage  represented  by  the  Frassi 
embryo  a  plate  of  mesoderm  has  formed  on  either  side  of  the  chorda 
endoderm,  and  that  in  a  later  stage,  represented  by  the  Kromer 
embryo  A76,  differentiation  occurs  in  these  plates  leading  to  the 


8o  THE    MESODERMIC    SOMITES 

formation  of  mesodermic  somites.  These  make  their  appearance 
in  what  will  later  be  the  cervical  region  of  the  embryo  and  their 
formation  proceeds  backward  as  the  body  of  the  embryo  increases 
in  length.  A  longitudinal  groove  appears  on  the  dorsal  surface  of 
each  lateral  plate  of  mesoderm,  marking  off  the  more  median 
thicker  portion  from  the  lateral  parts  (Fig.  49),  which  from  this 
stage  onward  may  be  termed  the  ventral  mesoderm.  The  median  or 
dorsal  portions  then  become  divided  transversely  into  a  number  of 
more  or  less  cubical  masses  which  are  termed  the  protovertebrce,  or, 


Fig.  49. — Transverse  Section  through  the   Second  Mesodermic  Somite  of 
Sheep  Embryo  3  mm.  Long. 
am,  Amnion;  en,  endoderm;  I,  intermediate  cell-mass;  mg,  medullary  groove;  ms, 
mesodermic  somite;  so,  somatic  and  sp,  splanchnic  layers  of  the  ventral  mesoderm. 
• — (Bonnet.) 

better,  mesodermic  somites  (Fig.  49,  ms).  The  cells  of  the  somites 
and  of  the  ventral  mesoderm,  are  at  first  stellate  in  form,  but  later 
become  more  spindle-shaped,  and  those  near  the  center  of  each 
somite  and  those  of  the  ventral  mesoderm  arrange  themselves  in 
regular  layers  so  as  to  enclose  cavities  which  appear  in  these  regions 
(Fig.  49).  Each  original  lateral  plate  of  gastral  mesoderm  thus 
becomes  divided  longitudinally  into  three  areas,  a  more  median 
area  composed  of  mesodermic  somites,  lateral  to  this  a  narrow  area 
underlying  the  original  longitudinal  groove  which  separated  the 
somite  area  from  the  ventral  mesoderm  and  which  from  its  position 
is  termed  the  intermediate  cell-mass  (Fig.  49,  /),  and,  finally,  the 
ventral  mesoderm.  This  last  portion  is  now  divided  into  two  lay- 
ers, the  dorsal  of  which  is  termed  the  somatic  mesoderm,  while  the 


THE   MESODERMIC    SOMITES  8 1 

ventral  one  is  known  as  the  splanchnic  mesoderm  (Fig.  49,  so  and  sp ; 
and  Fig.  50) ,  the  cavity  which  separates  these  two  layers  being  the 
embryonic  body-cavity  or  pleuro peritoneal  cavity  (coelom),  which 
will  eventually  give  rise  to  the  pleural,  pericardial  and  peritoneal 
cavities  of  the  adult  as  well  as  the  cavity  of  each  tunica  vaginalis 
testis.  In  the  early  stages  of  development  this  cavity  is  in  wide 
communication  with  the  extra-embryonic  coelom,  but  later  this 
communication  is  interrupted  (see  p.  89). 


Pig.  50. — Transverse  Section  of  an  Embryo  of  2.5  mm.  (See  Pig.  54)  showing 
ON  either  side  of  the  Medullary  Canal  a  Mesodermic  Somite,  the  Inter- 
mediate Cell-mass,  and  the  Ventral  Mesoderm. — (von  Lenhossek.) 

Beginning  in  the  neck  region,  the  formation  of  the  mesodermic 
somites  proceeds  posteriorly  until  finally  there  are  present  in  the 
human  embryo  thirty-eight  pairs  in  the  neck  and  trunk  regions  of 
the  body,  and,  in  addition,  a  certain  number  are  developed  in  what 
is  later  the  occipital  region  of  the  head.  Exactly  how  many  of 
these  occipital  somites  are  developed  is  not  known,  but  in  the  cow 
four  have  been  observed,  and  there  are  reasons  for  believing  that 
the  same  number  occurs  in  the  human  embryo. 

In  the  lower  vertebrates  a  number  of  cavities  arranged  in  pairs  occur 
in  the  more  anterior  portions  of  the  head  and  have  been  homologized 


82 


THE   MESODERMIC    SOMITES 


with  mesodermic  somites.  Whether  this  homology  be  perfectly  cor- 
rect or  not,  these  head-cavities,  as  they  are  termed,  indicate  the  ex- 
istence of  a  division  of  the  head  mesoderm  into  somites,  and  although 
practically  nothing  is  known  as  to  their  existence  in  the  human  embryo, 
yet,  from  the  relations  in  which  they  stand  to  the  cranial  nerves  and 
musculature  in  the  lower  forms,  there  is  reason  to  suppose  that  they 
are  not  entirely  unrepresented. 


.^!3^ 


Fig.  si. — Transverse  Section  of  an  Embryo  of  4.25  mm.  at  the  Level  of  the 

Arm  Rudiment. 
A,  Axial  mesoderm  of  arm;  Am,  amnion;  il,  inner  lamella  of  myotome;  M,  myo- 
tome; me,  splanchnic  mesoderm;  ol,  outer  lamella  of  myotome;  Pn,  place  of  origin 
of  pronephros;  5,  sclerotome;  S^,  defect  in  wall  of  myotome  due  to  separation  of 
the  sclerotome;  st,  stomach;  Vu,  umbilical  vein. — (Kollmann.) 


I'he  mesodermic  somites  in  the  earliest  human  embryos  in 
which  they  have  been  observed  contain  a  completely  closed  cavity, 
and  this  is  true  of  the  majority  of  the  somites  in  such  a  form  as  the 
sheep.  In  the  four  first-formed  somites  in  this^species,  however, 
the  somite  cavity  is  at  first  continuous  with  the  pleuroperitoneal 


THE    MESODERMIC    SOMITES  83 

cavity  and  only  later  becomes  separated  from  it,  and  in  lower 
vertebrates  this  continuity  of  the  somite  cavities  with  the  general 
body-cavity  is  the  rule.  The  somite  cavities  are  consequently  to 
be  regarded  as  portions  of  the  general  pleuroperitoneal  cavity 
which  have  secondarily  been  separated  off.  They  are,  however, 
of  but  short  duration  and  early  become  filled  up  by  spindle-shaped 
cells  derived  from  the  walls  of  the  somites,  which  themselves  under- 
go a  differentiation  into  distinct  portions.  The  cells  of  that  por- 
tion of  the  wall  of  each  somite  which  is  opposite  the  notochord 
become  spindle-shaped  and  grow  inward  toward  the  median  line 
to  surround  the  notochord  and  central  nervous  system,  and  give 
rise  eventually  to  the  lateral  half  of  the  body  of  a  vertebra  and 
the  corresponding  portion  of  a  vertebral  arch.  This  portion  of 
the  somite  is  termed  a  sclerotome  (Fig.  51,  S),  and  the  remainder 
forms  a  muscle  plate  or  myotome  (M)  which  is  destined  to  give  rise 
to  a  portion  of  the  voluntary  musculature  of  the  body.  The 
outer  wall  of  the  somite  has  been  generally  believed  to  take  part 
in  the  formation  of  the  cutis  layer  of  the  integument  and  hence 
has  been  termed  the  cutis  plate  or  dermatome,  but  it  seems  probable 
that  in  mammals,  it  becomes,  transformed  into  muscular  tissue. 

The  intermediate  cell-mass  in  the  human  embryo,  as  in  lower 
forms,  partakes  of  the  transverse  divisions  which  separate  the 
individual  mesodermic  somites.  From  one  portion  of  the  tissue 
in  most  of  the  somites  (Fig.  51,  Pn)  the  provisional  kidneys  or 
Wolffian  bodies  develop,  this  portion  of  each  mass  being  termed 
a  nephrotome,  while  the  remaining  portion  gives  rise  to  a  mass  of 
cells  showing  no  tendency  to  arrange  themselves  in  definite  layers 
and  constituting  that  form  of  mesoderm  which  has  been  termed 
mesenchyme  (see  p.  64).  These  mesenchymatous  masses  become 
converted  into  connective  tissues  and  blood-vessels. 

The  ventral  mesoderm  in  the  neck  and  trunk  regions  never 
becomes  divided  transversely  into  segments  corresponding  to  the 
mesodermic  somites,  differing  in  this  respect  from  the  other  por- 
tions of  the  lateral  mesoderm.  In  the  head,  however,  that  portion 
of  the  middle  layer  which  corresponds  to  the  ventral  mesoderm  of 
the  trunk  does  undergo  a  division  into  segments  in  connection 


84  THE    MESODERMIC    SOMITES 

with  the  development  of  the  branchial  arches  and  clefts  (see  p.  93). 
A  consideration  of  these  segments,  which  are  known  as  the 
branchiomeres,  may  conveniently  be  postponed  until  the  chapters 
dealing  with  the  development  of  the  cranial  muscles  and  nerves, 
and  in  what  follows  here  attention  will  be  confined  to  what  occurs 
in  the  ventral  mesoderm  of  the  neck  and  trunk. 

Its  splanchic  layer  (Fig.  52,  vm),  applies  itself  closely  to  the 
endodermal  digestive  tract,  which  is  constricted  off  from  the  dorsal 
portion  of  the  yolk-sac,  and  becomes  converted  into  mesenchyme 
out  of  which  the  muscular  coats  of  the  digestive  tract  develop. 
The  cells  which  line  the  pleuroperitoneal  cavity,  however,  retain 
their  arrangement  in  a  layer  and  form  a  part  of  the  serous  lining  of 
the  peritoneal  and  other  serous  cavities,  the  remainder  of  the  lining 
being  formed  by  the  corresponding  cells  of  the  somatic  layer;  and 
in  the  abdominal  region  the  superficial  cells,  situated  near  the  line 
where  the  splanchnic  layer  passes  into  the  somatic,  and  in  close 
proximity  to  the  nephrotome  of  the  intermediate  cell-mass,  be- 
come columnar  in  shape  and  are  converted  into  reproductive  cells. 

The  somatic  layer,  if  traced  peripherally,  becomes  continuous 
at  the  sides  with  the  layer  of  mesoderm  which  lines  the  outer 
surface  of  the  amnion  (Fig.  51)  and  posteriorly  with  the  mesoderm 
of  the  belly-stalk.  T  hat  portion  of  it  which  lies  within  the  body 
of  the  embryo,  in  addition  to  giving  rise  to  the  serous  lining  of  the 
parietal  layer  of  the  pleuroperitoneum,  becomes  converted  into 
mesenchyme,  which  for  a  considerable  length  of  time  is  clearly 
differentiated  into  two  zones,  a  more  compact  dorsal  one  which 
may  be  termed  the  somatic  layer  proper,  and  a  thinner,  more 
ventral  vascular  zone  which  is  termed  the  membrana  reuniens 
(Fig.  52).  In  the  earlier  stages  the  somatic  layer  proper  does  not 
extend  ventrally  beyond  the  line  which  passes  through  the  limb 
buds  and  it  grows  out  into  these  buds  to  form  an  axial  core  for 
them,  Iq  which  later  the  skeleton  of  the  limb  forms.  The  remain- 
der of  the  mesoderm  lining  the  sides  and  ventral  portions  of  the 
body-wall  is  at  first  formed  from  the  membrana  reuniens,  but  as 
development  proceeds  the  somatic  layer  gradually  extends  more 
ventrally  and  displaces,  or,  more  properly  speaking,  assimilates 


THE    MESODERMIC    SOMITES 


8s 


into  itself,  the  membrana  reuniens  until  finally  the  latter  has 
completely  disappeared. 

It  is  to  be  noted  that  no  part  of  the  voluntary  musculature 
of  the  lateral  and  ventral  walls  of  the  neck  and  trunk  is  derived 
from  the  somatic  layer;  it  is  formed  entirely  from  the  myotomes 
which  gradually  extend  ventrally  (Fig.  52)  and  finally  come  into 
contact  with  their  fellows  of  the  opposite  side  in  the  mid-ventral 
line.  Whether  the  voluntary  musculature  of  the  limbs  is  also 
derived  from  the  myotomes  is  at  present  doubtful.  It  has  been 
very  generally  believed  that  the  myotomes  in  their  growth  ven- 
trally sent  prolongations  into  the  limb  buds  which  invested  the 


Fig,  52. — Diagrams  Illustrating  the  History  of  the  Gastral   Mesoderm. 
dM,  dorsal  portion  of  myotome;  gr,  genital  ridge;  /,  intestine;  M,  myotome,  mr, 
membrana  reuniens;    N,  nervous  system;  SC,  sclerotome;  Sm,  somatic  mesoderm; 
vm,  splanchnic  mesoderm;  vM,  ventral  portion  of  myotome;  Wd,  Wolffian  duct. 


axial  core  of  mesenchyme  and  eventually  gave  rise  to  the  voluntary 
muscles.  The  actual  existence  of  the  prolongations  of  the  myo- 
tomes and  their  conversion  into  the  limb  musculature  has,  how- 
ever, not  yet  been  observed  and  it  is  quite  probable  that  the  limb 
musculature  may  be  derived  from  the  axial  core  of  somatic  meso- 
derm from  which  the  limb  skeleton  develops. 

The  appearance  of  the  mesodermic  somites  is  an  important 
phenomenon  in  the  development  of  the  embryo,  since  it  influences 
fundamentally  the  future  structure  of  the  organism.     If  each  pair 


86  THE    MESODERMIC    SOMITES 

of  mesodermic  somites  be  regarded  as  a  structural  unit  and  termed 
a  metamere  or  segment,  then  it  may  be  said  that  the  body  is  com- 
posed of  a  series  of  metameres,  each  more  or  less  closely  resembling 
its  fellows,  and  succeeding  one  another  at  regular  intervals.  Each 
somite  differentiates,  as  has  been  stated,  into  a  sclerotome  and  a 
myotome,  and,  accordingly,  there  will  primarily  be  as  many  verte- 
brae and  muscle  segments  as  there  are  mesodermic  somites,  or,  in 
other  words,  the  axial  skeleton  and  the  voluntary  muscles  of 
the  trunk  are  primarily  metameric.  Nor  is  this  all.  Since  each 
metamere  is  a  distinct  unit,  it  must  possess  its  own  supply  of  nutri- 
tion, and  hence  the  primary  arrangement  of  the  blood-vessels  is 
also  metameric,  a  branch  passing  off  on  either  side  from  the  main 
longitudinal  arteries  and  veins  to  each  metamere.  And,  further, 
each  pair  of  muscle  segments  receives  its  own  nerves,  so  that  the 
arrangement  of  the  nerves,  again,  is  distinctly  metameric. 

It  is  to  be  noted  that  this  metamerism  is  essentially  resident 
in  the  dorsal  mesoderm,  the  segmentation  shown  by  structures 
derived  from  other  embryonic  tissues  being  secondary  and  asso- 
ciated with  the  relations  of  these  structures  to  the  mesodermic 
somites.  The  metamerism  is  most  distinct  in  the  neck  and  trunk 
regions,  and  at  first  only  in  the  dorsal  portions  of  these  regions,  the 
ventral  portions  showing  metamerism  only  after  the  extension 
into  them  of  the  myotomes.  But  there  is  clear  evidence  that  the 
arrangement  extends  also  into  the  head,  and  that  a  portion  of  its 
mesoderm  is  to  be  regarded  as  composed  of  metameres.  It  has 
been  seen  that  in  the  notochordal  region  of  the  head  of  lower 
vertebrates  mesodermic  somites  are  present,  while  anteriorly  in 
the  praechordal  region  there  are  head-cavities  which  resemble 
closely  the  mesodermic  somites,  and  are  probably  directly  com- 
parable to  the  somites  of  the  trunk.  There  is  reason,  therefore, 
for  believing  that  the  fundamental  arrangement  of  the  dorsal 
mesoderm  in  all  parts  of  the  body  is  metameric,  but  though  this 
arrangement  is  clearly  defined  in  early  embryos,  it  loses  distinct- 
ness in  later  periods  of  development.  But  even  in  the  adult  the 
original  metamerism  is  clearly  indicated  in  the  arrangement  of 
the  nerves  and  of  parts  of  the  axial  skeleton,  and  careful  study 


LITERATURE  87 

frequently  reveals  indications  of  it  in  highly  modified  muscles  and 
blood-vessels. 

In  the  head  the  development  of  the  branchial  arches  and  clefts 
produces  a  series  of  parts  presenting  many  of  the  peculiarities  of 
metameres,  and,  indeed,  it  has  been  a  very  general  custom  to 
regard  them  as  expressions  of  the  general  metamerism  which  pre- 
vails throughout  the  body.  It  is  to  be  noted,  however,  that  they 
are  produced  by  the  segmentation  of  the  ventral  mesoderm,  a 
structure  which  in  the  neck  and  trunk  regions  does  not  share  in 
the  general  metamerism,  and,  furthermore,  recent  observations 
on  the  cranial  nerves  seem  to  indicate  that  these  branchiomeres 
cannot  be  regarded  as  portions  of  the  head  metameres  or  even  as 
structures  comparable  to  these.  They  represent,  more  probably, 
a  second  metamerism  superposed  upon  the  more  general  one,  or, 
indeed,  possibly  more  primitive  than  it,  but  whose  relations  can 
only  be  properly  understood  in  connection  with  a  study  of  the 

cranial  nerves. 

LITERATURE 

In  addition  to  many  of  the  papers  cited  in  the  list  at  the  close  of  Chapter  II,  the 
following  may  be  mentioned: 
C.  R.  Bardeen:  "The  Development  of  the  Musculature  of  the  Body  Wall  in  the 

Pig,  etc.,"  Johns  Hopkins  Hosp.  Rep.,  ix,  1900. 
T.  H.  Bryce  and  J.  H.  Teacher:  "  Contributions  to  the  Study  of  the  Early  Develop- 
ment and  Imbedding  of  the  Human  Ovum,"  Glasgow,  1908. 
A.  C.  F.  Eternod:  "Communication  sur  un  oeuf  humain  avec  embryon  excessive- 

ment  jeune,"  Arch.  Hal.  de  Biologie,  xxii,  1895. 
A.  C.  F.  Eternod:  "11  y  a  un  canal  notochordal  dans  I'embryon  humain,"  Anal. 

Anzeiger,  xvi,  1899. 
A.  C.  F.  Eternod:  "Les  premiers  stades  du  developpement  de  I'oeuf  humain," 

Trans.  Internal.  Congr.  Med.  London,  Sect.  I,  pt.  i,  1913. 
Fetzer:  "Ueber  ein  durch  Operation  gewonnenes  menschliches  Ei  das  in  seiner 

Entwickelung  etwa  dem  Petersschen  Ei  entspricht,"  Verh.  Anat.  Gesellschaft, 

XXIV,  1 901. 
L.  Frassi:  "Weitere  Ergebnisse  des  Studiums  eines  jungen  menschlichen  Eies  in 

situ,"  Arch.f.  mikr.  Anat.,  lxxi,  1908. 
0.  Grosser:  " Ein  menschlicher  Embryo mit  Chordakanal,"  Anat.  Hefte,XLVii,  19 13. 
W.  Heape:  "The  Development  of  the  Mole  (Talpa  Europaea),"  Quarterly  Journ. 

Micros c.  Science,  xxvii,  1887. 
M.  Herzog:  "A  contribution  to  our  Knowledge  of  the  Earliest  Known  Stages  of 

Placentation  and  Embryonic  Development  in  Man,"  Amer.  Journ.  Anat.,  ix, 

1909. 


88  LITERATURE 

P.  Jung:  "Beitrage  zur  fruhesten  Eieinbettung  beim  menschlichen  Weibe,"  Berlin, 

1908. 
F.  Keibel:  "Zur  Entwickelungsgeschichte  der  Chorda  bei  Saugern  (Meerschein- 

chen  und  Kaninchen),"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  1889. 
S.  Kaestner:  "Ueber  die  Bildung  von  animalen  Muskelfasern  aus  dem  Urwirbel," 

Arch,  fur  Anat.  undPhys.,  Anat.  Abth.,  SuppL,  1890. 
J.  Kollmann:  "Die  Rumpfsegmente  menschlicher  Embryonen  von  13  bis  35  Urwir- 

beln,"  Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1891. 
Linzenmeier:  "Ein  junges  menschliches  Eies  in  situ," -4f<;//. /«>  G'>'Mflf^,  cii,  i9i4> 
H.  Peters:  "Ueber  die  Einbettung  des  menschlichen  Eies  und  das  fruheste  bisher 

bekannte  menschliche  Placentarstadium,"  Leipzig  und  Wien,  1899. 
F.    Graf  von   Spee:  "  Beobachtungen  an  einer   menschlichen   Keimscheibe   mit 

offener  Medullarrinne  und  Canalis  neurentericus,"  Arch.f.  Anat.  u.  Phys.,  Anat. 

Abth.,  1889. 
F.  Graf  von  Spee:  "Ueber  friihe  Entwicklungsstufen  des  menschlichen  Eies," 

Arch.  f.  Anat.  u.  Phys.,  Anat.  Abth.,  1896. 
H.  Strahl  and  R.  Beneke:  "Ein  junger  menschlicher  Embryo,"  Wiesbaden,  1910. 
J.  W.  VAN  Wijhe:  "Ueber  die  Mesoderm segmente  des  Rumpfes  und  die  Entwick- 

lung  des  Excretionsystems  bei  Selachiern,"  Archiv  fiir  mikrosk.  Anat.,  xxxiii, 

1889. 
K.  W.  Zimmermann:  "Ueber  Kopfhohlenrudimente  beim  Menschen,"  Archiv  fiir 

mikrosk.  Anat.,  Liii,  1898. 


CHAPTER  IV 

THE  DEVELOPMENT  OF  THE  EXTERNAL  FORM  OF 
THE  HUMAN  EMBRYO 

In  the  preceding  chapter  descriptions  have  been  given  of 
human  embryos  representing  the  earHer  known  stages  and  the 
development  of  the  general  form  of  the  human  embryo  has  been 
traced  up  to  the  time  when  the  mesodermic  somites  have  made 
their  appearance.  It  will  now  be  convenient  to  continue  the  his- 
tory of  the  general  development  up  to  the  stage  when  the  embryo 
becomes  a  fetus. 

In  the  earlier  stages,  that  is  to  say  up  to  that  represented  by 
the  Eternod  embryo  (Fig.  44),  the  embryonic  disk  may  be  de- 
scribed as  floating  upon  the  surface  of  the  yolk-sac,  and  while  this 
description  still  holds  good  for  the  Eternod  embryo  a  distinct 
groove  may  be  seen  in  that  embryo  between  the  peripheral  portions 
of  the  embryonic  disk  and  the  upper  part  of  the  sac.  This  groove*" 
marks  the  beginning  of  the  separation  or  constriction  of  the  em- 
bryo from  the  yolk-sac,  the  result  of  which  is  the  transformation 
of  the  discoidal  embryonic  portion  of  the  embryonic  disk  into  a 
cylindrical  structure.  Primarily  this  depends  upon  the  deepening 
of  the  furrow  which  surrounds  the  embryonic  area,  the  edges  of 
this  area  being  thus  bent  in  on  all  sides  toward  the  yolk-sac.  This 
bending  in  proceeds  most  rapidly  at  the  anterior  end  of  the  body, 
as  shown  in  the  diagrams  (Fig.  53),  and  less  rapidly  at  the  pos- 
terior end  where  the  belly-stalk  is  situated,  and  produces  a  con- 
striction of  the  yolk-sac,  the  portion  of  this  structure  nearest  the 
embryonic  disk  becoming  enclosed  within  the  body  of  the  embryo 
to  form  the  digestive  tract,  while  the  remainder  is  converted  into 
a  pedicle-like  portion,  the  yolk-stalk,  at  the  extremity  of  which  is 
the  yolk-vesicle.  The  further  continuance  of  the  folding  in  of  the 
edges  of  the  embryonic  area  leads  to  an  almost  complete  closing 

89 


go 


DEVELOPMENT  OF  EXTERNAL  FORM 


in  of  the  embryonic  coelom  and  reduces  the  opening  through  which 
the  yolk-stalk  and  belly-stalk  communicate  with  the  embryonic 
tissues  to  a  small  area  known  as  the  umbilicus. 

In  the  Kromer  embryo  Klb  (Fig.  45)  this  separation  of  the 
embryo  proper  from  the  yolk-sac  has  proceeded  to  such  an  extent 
that  both  extremities  of  the  embryonic  disk  are  free  from  the  yolk- 
sac,  and  the  anterior  extremity  is  bent  ventrally  almost  at  a  right 
angle  to  the  rest  of  the  disk,  producing  what  is  termed  the  vertex 
bend,  sl  feature  characteristic  of  all  later  embryos.     The  marked 


Fig.  S3. — Diagrams  Illustrating  the  Constriction  of  the  Embryo  from  the 

Yolk-sac. 

A  and  C  are  longitudinal,  and  B  and  D  transverse  sections.     B  is  drawn  to  a  larger 

scale  than  the  other  figures. 

development  in  this  embryo  of  the  medullary  folds  and  the  occur- 
rence of  mesodermic  somites  have  already  been  mentioned  (p.  75). 
Somewhat  more  advanced  is  the  Bulle  embryo  described  by 
Kollmann  and  shown  from  the  side  and  dorsally  in  Fig.  54,  the 
greater  part  of  the  yolk-sac  having  been  removed  as  well  as  the 
most  of  the  amnion.  The  embryo  measured  about  2.5  mm.  in 
length  and  showed  a  considerable  increase  in  the  number  of  meso- 
dermic somites,  there  being  about  fourteen  of  them  on  either  side. 
Posteriorly  the  medullary  groove  has  become  converted  into  a 
medullary  canal  by  the  medullary  folds  meeting  over  it  and  fusing, 


DEVELOPMENT  OF  EXTERNAL  FORM 


91 


but  anteriorly  it  is  still  open.  The  vertex  bend  is  well  marked  and 
immediately  behind  the  tip  of  the  head,  on  the  ventral  surface  of 
the  body,  there  may  be  seen  a  well-marked  depression,  the  oral 
fossa,  between  which  and  the  anterior  surface  of  the  yolk-sac  is  a 


am-<: 


Fig.  54. — Embryo  2.5  mm.  Long. 
am.  Amnion;  B,  belly-stalk;  h,  heart;  M,  closed,  and  M',  still  open  portions  of  the 
medullary  groove;  Om,  vitelline  vein;  OS,  oral  fossa;  Y,  yolk-sac. — (Kollmann.)   g 

rounded  elevation  due  to  the  formation  of  the  heart.  Attention 
may  be  called  to  the  fact  that  the  position  of  this  organ  is  far 
forward  of  that  which  it  will  eventually  occupy,  so  that  it  must 
undergo  a  marked  retrogression  during  later  development. 


92 


DEVELOPMENT  OF  EXTERNAL  FORM 


Pig.  55.-^Embryo  Lr,  4.2  mm.  Long. 
am,  Amnion;  au,  auditory  capsule;  B,  belly-stalk;  h,  heart;  LI,  lower,  and  ul,  upper 
limb;  Y,  yolk-sac. — (His.) 


DEVELOPMENT  OF  EXTERNAL  FORM  93 

As  an  example  of  a  later  stage  of  development  the  embryo  Lr  of 
His,  measuring  4.2  mm.  in  length,  may  be  taken  (Fig.  55).  In  this 
the  constriction  of  the  yolk-sac  has  progressed  so  far  that  its 
proximal  portion  may  now  be  spoken  of  as  the  yolk-stalk.  The 
mesodermic  somites  have  undergone  a  further  increase  and  have 
almost  reached  their  final  number,  the  vertex  bend  has  become 
still  more  pronounced  and  the  medullary  groove,  throughout  its 
entire  length,  has  been  converted  into  the  medullary  canal,  which, 
anteriorly,  shows  distinct  enlargements  and  constrictions  which 
foreshadow  various  portions  of  the  future  brain.  The  auditory 
organ,  which  made  its  appearance  in  earlier  stages,  has  now  become 
quite  distinct,  and  a  lateral  bulging  of  the  most  anterior  portion 
of  the  head  indicates  the  position  of  the  future  eye. 

In  addition  certain  other  important  features  have  now  ap- 
peared. Thus,  about'  opposite  the  heart  a  second  bend,  the  nape 
bend,  is  becoming  visible  on  the  dorsal  surface  of  the  body  and 
toward  the  posterior  end  a  distinct  sacral  bend  is  evident.  Sec- 
ondly, a  little  posterior  to  the  level  of  the  nape  bend  a  slight 
elevation  is  to  be  seen  on  the  side  of  the  body;  this  is  the  limb  bud 
for  the  upper  limb  and  a  corresponding,  though  smaller,  elevation 
in  the  region  of  the  sacral  bend  represents  the  lower  limb. 

Thirdly,  three  grooves  having  a  dorso-ventral  direction  have 
appeared  on  the  sides  of  what  will  be  the  future  pharyngeal  region. 
These  are  representatives  of  a  series  of  branchial  clefts,  structures 
that  are  of  great  morphological  importance  in  the  further  develop- 
ment inasmuch  as  they  determine  to  a  large  extent  the  arrange- 
ment of  various  organs  of  the  head  region.  They  represent  the 
clefts  which  exist  in  the  walls  of  the  pharynx  in  fishes,  through 
which  water,  taken  in  at  the  mouth,  passes  to  the  exterior,  bathing 
on  its  way  the  gill  filaments  attached  to  the  bars  or  arches,  as 
they  are  termed,  which  separate  successive  clefts.  Hence  the 
name  ''branchial"  which  is  appHed  to  them,  though  in  the  mam- 
mals they  never  have  respiratory  functions  to  perform,  but,  ap- 
pearing, persist  for  a  time  and  then  either  disappear  or  are  applied 
to  some  entirely  different  purpose.  Indeed,  in  man  they  are  never 
really  clefts  but  merely  grooves,  and  corresponding  to  each  groove 


94  DEVELOPMENT  OF  EXTERNAL  FORM 

in  the  ectoderm  there  is  also  one  in  the  subjacent  endoderm  of 
what  will  eventually  be  the  pharyngeal  region  of  the  digestive 
tract,  so  that  in  the  region  of  each  cleft  the  ectoderm  and  endo- 
derm are  in  close  relation,  being  separated  only  by  a  very  thin 

layer  of  mesoderm.  In  the  intervals 
between  successive  clefts  a  more 
considerable  amount  of  mesoderm  is 
present   (Fig.   56). 

In  the  human  embryo  four  clefts 
and    five   branchial   arches  develop 
on  each  side  of  the  body,  the  last 
arch  lying  posteriorly  to  the  fourth 
Fig.  56.-FL00R  OF  THE  Pharynx    cleft  and  not  being  very  sharply  de- 
OF  Embryo  b.  7  mm.  Long.       fined  along  its  posterior  margin. 

Ep,  Epiglottis;  Sp,  sinus  praecer- 

vicalis;  t\  tuberculum  impar;  t^  As  just  Stated,  the  clefts  are  nor- 

posterior  portions  of  the  tongue;    j^^Uy    merely   grooves,    and    in    later 

/.   II,   III,   and  IV,  branchial     J        <  ^       -^u         j- 

arches.— (His.)  development   either   disappear   or   are 

converted  into  special  structures. 
Occasionally,  however,  a  cleft  may  persist  and  the  thin  membrane 
which  forms  its  floor  may  become  perforated  so  that  an  opening  from 
the  exterior  into  the  pharynx  occurs  at  the  side  of  the  neck,  forming 
what  is  termed  a  branchial  fistula.  Such  an  abnormality  is  most  fre- 
quently developed  from  the  lower  (ventral)  part  of  the  first  cleft; 
normally  this  disappears,  the  upper  portion  of  the  cleft  persisting, 
however,  to  form  the  external  auditory  meatus  and  tympanic  cavity. 

A  further  stage  in  the  differentiation  of  these  clefts  and  arches 
is  shown  by  the  embryo  represented  in  Fig.  57.  The  nape  bend 
has  now  increased  to  such  an  exent  that  the  whole  anterior  part 
of  the  body  is  bent  at  a  right  angle  to  the  middle  part  and  the 
entire  embryo  is  coiled  in  a  spiral  manner.  The  limb  buds  are 
much  more  distinct  than  in  the  previous  stage  and  four  branchial 
arches  are  now  present;  the  second  and  third  have  become  more 
defined  and  a  strong  process  has  developed  from  the  dorsal  part 
of  the  anterior  border  of  the  first  one,  which  has  thus  become 
somewhat  A-shaped.  The  anterior  limb  of  each  A  is  destined  to 
give  rise  to  the  upper  jaw,  and  hence  is  known  as  the  maxillary 
process,  while  the  posterior  limb  represents  the  future  lower  jaw 
and  is  termed  the  mandibular  process. 


DEVELOPMENT  OF  EXIERNAL  FORM  95 

In  the  stage  represented  by  this  embryo  the  closing  in  of  the 
embryonic  ccelom  has  progressed  to  such  a  degree  that  only  a 
small  opening  is  left  in  the  ventral  body- wall  of  the  embryo 
through  which  the  yolk-stalk  and  its  accompanying  vessels  and  the 
belly-stalk  pass.  Indeed  the  margins  of  the  umbilicus  may  have 
begun  to  be  prolonged  outward  over  these  structures,  enclosing 
them  in  a  cylindrical  investment,  the  first  stage  of  what  will  later 
be  the  umbilical  cord  being  thus  established. 


Fig.  57. — Embryo  Backer  7.3  mm.  in  Length.    X  5. — {Keihel  and  Elze.) 

Leaving  aside  for  the  present  all  consideration  of  the  further 
development  of  the  limbs  and  branchial  arches,  the  further  evolu- 
tion of  the  general  form  of  the  body  may  be  rapidly  sketched.  In 
an  embryo  (Fig.  58)  from  Ruge's  collection,  described  and  figured 
by  His  and  measuring  9.1  mm.  in  length,*  the  prolongation  of  the 

*  This  measurement  is  taken  in  a  straight  line  from  the  most  anterior  portion  of 
the  nape  bend  to  the  middle  point  of  the  sacral  bend  and  does  not  follow  the  curva- 
ture of  the  embryo.  It  may  be  spoken  of  as  the  nape-rump  length  and  is  convenient 
for  use  during  the  stages  when  the  embryo  is  coiled  upon  itself. 


96 


DEVELOPMENT  OF  EXTERNAL  FORM 


margins  of  the  umbilicus  has  increased  until  more  than  half  the 
yolk-stalk  has  become  enclosed  within  the  umbilical  cord.  The 
nape  and  sacral  bends  are  still  very  pronounced,  although  the  em- 
bryo is  beginning  to  straighten  out  and  is  not  quite  so  much  coiled 
as  in  the  preceding  stage.  At  the  posterior  end  of  the  body  there 
has  developed  a  rather  abruptly  conical  tail  filament,  in  the  place  of 


Fig.  58. — Embryo  9.1  mm.  Long. 
LI,  Lower  limb;  U,  umbilical  cord;  Ul,  upper  limb;  Y,  yolk-sac- 


■{His.) 


the  blunt  and  gradually  tapering  termination  seen  in  earlier  stages, 
and  a  well-marked  rotundity  of  the  abdomen,  due  to  the  rapidly 
increasing  size  of  the  liver,  begins  to  become  evident. 

In  later  stages  the  enclosure  of  the  yolk-  and  belly-stalks  within 
the  umbilical  cord  proceeds  until  finally  the  cord  is  complete 
through  the  entire  interval  between  the  embryo  and  the  wall  of  the 
ovum.  At  the  same  time  the  straightening  out  of  the  embryo 
continues,  as  may  be  seen  in  Fig.  59  representing  the  embryo  xlv 


DEVELOPMENT  OP  EXTERNAL  FORM  97 

(Br2)  of  His,  which  shows  also,  both  in  front  of  and  behind  the 
neck  bend,  a  distinct  depression,  the  more  anterior  being  the 
occipital  and  the  more  posterior  the  nape  depression;  both  these 
depressions  are  the  indications  of  changes  taking  place  in  the 
central  nervous  system.  The  tail  filament  has  become  more 
marked,  and  in  the  head  region  a  slight  ridge  surrounding  the 
eyeball  and  marking  out  the  conjunctival  area  has  appeared;  a 
depression  anterior  to  the  nasal  fossae  marks  off  the  nose  from  the 
forehead ;  and  the  external  ear,  whose  development  will  be  consid- 


FiG.  59. — Embryo  Br2,  13.6  mm.  Long. — (His.) 

ered  later  on,  has  become  quite  distinct.     This  embryo  had  a 
nape-rump  length  of  13.6  mm. 

In  the  embryos  S2  and  L3  (Fig.  60,  A  and  B)  of  His'  collection 
the  straightening  out  of  the  nape  bend  is  proceeding,  and  indeed  is 
almost  completed  in  embryo  L3,  which  begins  to  resemble  closely 
the  fully  formed  fetus.  The  tail  filament,  somewhat  reduced  in 
size,  still  persists  and  the  rotundity  of  the  abdomen  continues  to  be 
well  marked.  The  neck  region  is  beginning  to  be  distinguishable 
in  embryo  S2  and  in  embryo  L3  the  eyelids  have  appeared  as  slight 
folds  surrounding  the  conjunctival  area.  The  nose  and  forehead 
are  clearly  defined  by  the  greater  development  of  the  nasal  groove 


98 


DEVELOPMENT  OF  EXTERNAL  FORM 


and  the  nose  has  also  become  raised  above  the  general  surface  of  the 
face,  while  the  external  ear  has  almost  acquired  its  final  fetal  form. 
These  embryos  measure  respectively  about  15  and  17.5  mm.  in 
length.* 

Finally,  an  embryo — again  one  of  those  described  by  His, 
namely,  his  Wt,  having  a  length  of  23  mm. — may  be  figured 
(Fig.  61)  as  representing  the  practical  acquisition  of  the  fetal 


Pig.  60. 


-A,  Embryo  S2,  15  mm.    Long  (showing  Ectopia  of  the  Heart);  B, 
Embryo  L2,  17.5  mm.  Long. — {His.) 


form.  This  embryo  dates  from  about  the  end  of  the  second 
month  of  pregnancy,  and  from  this  period  onward  it  is  proper  to 
use  the  term  fetus  rather  than  that  of  embryo.  The  changes 
which  have  been  described  in  preceding  stages  are  now  complete 
and  it  remains  only  to  be  mentioned  that  the  caudal  filament, 
which  is  still  prominent,  gradually  disappears  in  later  stages, 
becoming,  as  it  were,  submerged  and  concealed  beneath  adjacent 
parts  by  the  development  of  the  buttocks.     The  incompleteness 

*  The  embryo  S2  presents  a  slight  abnormality  in  the  great  projection  of  the 
heart,  but  otherwise  it  appears  to  be  normal. 


DEVELOPMENT    OF   THE   BRANCHIAL   ARCHES 


99 


of  the  development  of  these  regions  in  embryo  Wt  is  manifest, 
not  only  from  the  projection  of  the  tail  filament,  but  also  from  the 
external  genitalia  being  still  largely  visible  in  a  side  view  of  the- 
embryo,  a  condition  which  will  disappear  in  later  stages. 


Pig.  6i. — Embryo  Wt,  23  mm.  Long. — (His.) 


The  Later  Development  of  the  Branchial  Arches,  and  the 
Development  of  the  Face. — In  the  embryo  shown  in  Fig.  57,  the 
four  branchial  clefts  and  five  arches  which  develop  in  the  human 
embryo  are  visible  in  surface  view^s^  but  in^  the  Euge  embryo  (Fig. 
58)  it  will  be  noticed  t-i^t  qnly^the  firt^t  two  arches,  the  first  with  a 
well-developed  maxillary  process,  and  the  cleft  separating  them 
can  be  distinguished.  Tllis  ii  diie  to'b  sinti>ig  ihw^id  of  the  region 
occupied  by  the  tKiee  posterior  arches  so  that  a  triangular  depres- 


lOO  DEVELOPMENT    OF    THE   BRANCHIAL   ArCHES 

sion,  the  sinus  prcBcervicalis,  is  formed  on  each  side  of  what  will 
later  become  the  anterior  part  of  the  neck  region.  This  is  well 
shown  in  an  embryo  (Br2)  described  by  His  which  measured  6.9 
mm.  in  length  and  of  which  the  anterior  portion  is  shown  in  Fig. 
62.  The  anterior  boundary  of  the  sinus  (ps)  is  formed  by  the 
posterior  edge  of  the  second  arch  and  its  posterior  boundary  by 
the  thoracic  wall,  and  in  later  stages  these  two  boundaries  gradu- 
ally approach  one  another  so  as  first  of  all  to  diminish  the  opening 
into  the  sinus  and  later  to  completely  obliterate  it  by  fusing  to- 


pic. 62. — Head  of  Embryo  of  6.9  mm. 
na.  Nasal  pit;  ps,  praecervical  sinus. — (His.) 

gether,  the  sinus  thus  becoming  converted  into  a  completely  closed 
cavity  whose  floor  is  formed  by  the  ectoderm  covering  the  three 
posterior  arches  and  the  clefts  separating  these.  This  cavity 
eventually  undergoes  degeneration,  no  traces  of  it  occurring  nor- 
mally in  the  ad«ltf,  although  certain  cysts  occasionally  observed 
in  the  sides  of  th&nfcl;»may  rj^iesent  .p^rsjstipg  portions  of  it. 

A  somewhat  aimijar  process  results  in  tke  clgsure  of  the  ventral 
portion  of  the  first  del t,*  a  fold  grov;ing  backward  from  the  pos- 

*  See  page  94,  small  type. 


DEVELOPMENT    OF    THE   BRANCHIAL   ARCHES  lOI 

terior  edge  of  the  j&rst  arch  and  fusing  with  the  ventral  part  of 
the  anterior  border  of  the  second  arch.  The  upper  part  of  the 
cleft  persists,  however,  and,  as  already  stated,  forms  the  external 
auditory  meatus,  the  pinna  of  the  ear  being  developed  from  the 
adjacent  parts  of  the  first  and  second  arches  (Figs.  59  and  60). 

The  region  immediately  in  front  of  the  first  arch  is  occupied 
by  a  rather  deep  depression,  the  oral  fossa,  whose  early  develop- 
ment has  already  been  noticed.     In  an  embryo  measuring  8  mm. 


Fig.  63. — Face  of  Embryo  of  8  mm. 
mxp.  Maxillary  process;  np,  nasal  pit;  os,  oral  fossa;  pg,  processus  globularis. — {His. 

in  length  (Fig.  63)  the  fossa  {os)  has  assumed  a  somewhat  irregular 
quadrilateral  form.  Its  posterior  boundary  is  formed  by  the 
mandibular  processes  of  the  first  arch,  while  laterally  it  is  bounded 
by  the  maxillary  processes  {mxp)  and  anteriorly  by  the  free  edge 
of  a  median  plate,  termed  the  nasal  process,  which  on  either  side 
of  the  median  line  is  elevated  to  form  a  marked  protuberance, 
the  processus  globularis  {pg).  The  ventral  ends  of  the  maxillary 
processes  are  widely  separated,  the  nasal  process  and  the  proc- 
essus globulares  intervening  between  them,  and  they  are  also 
separated  from  the  globular  processes  by  a  deep  and  rather  wide 
groove  which  anteriorly  opens  into  a  circular  depression,  the 
nasal  pit  {np). 


102 


DEVELOPMENT    OF   THE   FACE 


Later  on  the  maxillary  and  globular  processes  unite,  obliterat- 
ing the  groove  and  cutting  off  the  nasal  pits — which  have  by  this 
time  deepened  to  form  the  nasal  fossae — from  direct  communica- 
tion with  the  mouth,  with  which,  however,  they  later  make  new 
communications  behind  the  maxillary  processes,  an  indication 
of  the  anterior  and  posterior  nares  being  thus  produced. 


Fig.  64. — Pace  of  Embryo  after  the  Completion  of  the  Upper  Jaw. — (His.) 

Occasionally  the  maxillary  and  globular  processes  fail  to  unite  on 
one  or  both  sides,  producing  a  condition  popularly  known  as  "harelip." 

At  the  time  when  this  fusion  occurs  the  nasal  fossae  are  widely 
separated  by  the  broad  nasal  process  (Fig.  64),  but  during  later 
development  this  process  narrows  to  form  the  nasal  septum  and 
is  gradually  elevated  above  the  general  surface  of  the  face  as 
shown  in  Figs.  59-61.  By  the  narrowing  of  the  nasal  process  the 
globular  processes  are  brought  nearer  together  and  form  the  por- 
tions of  the  upper  jaw  immediately  on  each  side  of  the  median 


DEVELOPMENT    OF   THE    LIMBS  I03 

line,  the  rest  of  the  jaw  being  formed  by  the  maxillary  processes. 
In  the  meantime  a  furrow  has  appeared  upon  the  mandibular 
process,  running  parallel  with  its  borders  (Fig.  60) ;  the  portion  o£ 
the  process  in  front  of  this  furrow  gives  rise  to  the  lower  lip  and 
is  known  as  the  lip  ridge,  while  the  portion  behind  the  furrow  be- 
comes the  lower  jaw  proper  and  is  termed  the  chin  ridge. 

The  Development  of  the  Limbs. — As  has  been  already  pointed 
out,  the  limbs  make  their  appearance  in  an  embryo  measuring 
about. 4  mm.  in  length  (Fig.  55)  and  are  at  first  bud-like  in  form. 
As  they  increase  in  length  they  at  first  have  their  long  axes  directed 
parallel  to  the  longitudinal  axis  of  the  body  and  become  somewhat 
flattened  at  their  free  ends,  remaining  cylindrical  in  their  proximal 
portions.  A  furrow  or  constriction  appears  at  the  junction  of  the 
flattened  and  cylindrical  portions  (F%.  58),  and  later  a  second  con- 
struction divides  the  cylindrical  portion  into  a  proximal  and  distal 
moiety,  the  three  segments  of  each  limb — the  arm,  forearm,  and 
hand  in  the  upper  limb,  and  the  thigh,  leg,  and  foot  in  the  lower — 
being  thus  marked  out.  The  digits  are  first  indicated  by  the  de- 
velopment of  four  radiating  shallow  grooves  upon  the  hand  and 
foot  regions  (Fig.  59),  and  a  transverse  furrow  uniting  the  proximal 
ends  of  the  digital  furrows  indicates  the  junction  of  the  digital 
and  palmar  regions  of  the  hand  or  of  the  toes  and  body  of  the 
foot.  After  this  stage  is  reached  the  development  of  the  upper 
limb  proceeds  more  rapidly  than  that  of  the  lower,  although  the 
processes  are  essentially  the  same  in  both  limbs.  The  digits  begin 
to  project  slightly,  but  are  at  first  to  a  very  considerable  extent 
united  together  by  a  web,  whose  further  growth,  however,  does 
not  keep  pace  with  that  of  the  digits,  these  thus  coming  to 
project  more  and  more  in  later  stages.  Even  in  comparatively 
early  stages  the  thumb,  and  to  a  somewhat  slighter  extent 
the  great  toe,  is  widely  separated  from  the  second  digit  (Figs.  60 
and  61). 

While  these  changes  have  been  taking  place  the  entire  limbs 
have  altered  their  position  with  reference  to  the -axis  of  the  body, 
being  in  stages  later  than  that  shown  in  Fig.  58  directed  ventrally 
so  that  their  longitudinal  axes  are  at  right  angles  to  that  of  the 


I04  DEVELOPMENT    OF   THE    LIMBS 

body.  From  the  figures  of  later  stages  it  may  be  seen  that  it  is  the 
thumb  (radial)  side  of  the  arm  and  the  great  toe  (tibial)  side  of  the 
leg  which  are  directed  forward;  the  plantar  and  palmar  surfaces 
of  the  feet  and  hands  are  turned  toward  the  body  and  the  elbow 
is  directed  outward  and  slightly  backward,  while  the  knee  looks 
outward  and  slightly  forward.  It  seems  proper  to  conclude  that 
the  radial  side  of  the  arm  is  homologous  with  the  tibial  side  of  the 
leg,  the  palmar  surface  of  the  hand  with  the  plantar  surface  of  the 
foot,  and  the  elbow  with  the  knee. 

The  limbs  are  not  yet,  however,  in  their  final  position  but  must 
undergo  a  second  alteration,  whereby  their  long  axes  again  become 
parallel  with  that  of  the  body.  This  is  accomplished  by  a  rotation 
of  the  limbs  around  axes  passing  through  the  shoulder-  and  hip- 
joints,  together  with  a  rotation  about  their  longitudinal  axes 
through  an  angle  of  90  degrees.  This  axial  rotation  of  the  upper 
limb  is,  however,  in  exactly  the  opposite  direction  to  that  of  the 
lower  limb  of  the  corresponding  side,  so  that  the  homologous 
surfaces  of  the  two  limbs  have  entirely  different  relations,  the 
radial  side  of  the  arm,  for  instance,  being  the  outer  side  while  the 
tibial  side  of  the  leg  is  the  inner  side,  and  whereas  the  palmar  sur- 
face of  the  hand  looks  ventrally,  the  plantar  surface  of  the  foot 
looks  dorsally. 

In  making  these  statements  no  account  is  taken  of  the  sec- 
ondary position  which  the  hand  may  assume  as  the  result  of  its 
pronation;  the  positions  given  are  those  assumed  by  the  limbs 
when  both  the  bones  of  their  middle  segment  are  parallel  to  one 
another. 

It  may  be  pointed  out  that  the  prevalent  use  of  the  physiological 
terms  flexor  and  extensor  to  describe  the  surfaces  of  the  limbs  has  a 
tendency  to  obscure  their  true  morphological  relationships.  Thus  if, 
as  is  usual,  the  dorsal  surface  of  the  arm  be  termed  its  extensor  surface, 
then  the  same  term  should  be  applied  to  the  entire  ventral  surface  of  the 
leg,  and  all  movements  of  the  lower  limb  ventrally  should  be  spoken  of 
as  movements  of  extension  and  any  movement  dorsally  as  movements 
of  flexion.  And  yet  a  ventral  movement  of  the  thigh  is  generally 
spoken  of  as  a  flexion  of  the  hip-joint,  while  a  straightening  out  of  the 
foot  upon  the  leg — that  is  to  say,  a  movement  of  it  dorsally — is  termed 
its  extension. 


AGE    OF   EMBRYO    AT   DIFFERENT    STAGES  I05 

The  Age  of  the  Embryo  at  Different  Stages. — The  age  of  an 

embryo  is  a  matter  of  considerable  moment  to  the  embryologist 
who  desires  to  trace  the  successive  stages  in  the  development  of 
any  organ.  In  the  case  of  the  human  embryo  an  exact  determina- 
tion of  the  age  is  somewhat  difficult,  since  in  the  majority  of  cases 
the  only  available  datum  from  which  it  may  be  estimated  is  the 
time  of  the  cessation  of  the  menses.  From  what  has  already 
been  said  (pp.  28,  37)  it  is  evident  that  this  menstruation  age  (Mall) 
can  only  be  approximative  to  the  actual  age,  which  should  date 
from  the  moment  of  fertilization.  The  available  evidence  (see  p. 
28)  indicates  that  ovulation  takes  place  at  some  time  in  the  inter- 
menstrual period,  on  the  average  about  the  middle  of  its  duration, 
but  since  this  duration  is  about  two  weeks  the  limits  of  variation 
from  the  average  must  be  quite  large,  too  large  to  be  of  much  value 
in  the  case  of  young  embryos,  where  a  day  means  much. 

The  earlier  attempts  at  estimating  the  ages  of  young  human 
embryos,  those  of  His  for  instance,  were  based  on  the  belief  that 
ovulation  took  place  as  a  rule  immediately  before  menstruation, 
and  if  fertilization  occurred  the  menses  were  omitted.  On  this 
basis  His  estimated  embryos  of  2.2  to  3.0  mm.  to  be  two  to  two 
and  a  half  weeks  old,  those  of  5.0  to  6.0  mm.  to  be  about  three  and 
one-half  weeks  and  those  of  lo.o  to  ii.o  mm.  to  be  about  four  and 
one-half  weeks.  It  is  certain,  however,  that  such  ages  are 
decidedly  too  low,  perhaps  by  as  much  as  a  week. 

A  small  number  of  cases  are  on  record  in  which  the  date  of  the 
coition  that  led  to  the  pregnancy  is  definitely  known.  This 
copulation  age  does  not  necessarily  give  the  exact  fertilization  age, 
but  it  is  probably  within  one,  or  at  most  two,  days  of  it  (see  p.  36). 
The  Bryce-Teacher  ovum,  with  an  embryo  measuring  0.15  mm. 
in  length,  was  the  result  of  a  coition  that  took  place  16  days  before 
the  ovum  was  aborted,  and  the  assumption  that  the  embryo  was 
about  two  weeks  old  cannot  be  far  astray.  Similarly  an  embryo 
described  by  Eternod  and  measuring  1.3  mm.  in  length  was  the 
result  of  a  single  coition  occurring  twenty-one  days  previously 
and  its  age  may  be  set  at  approximately  three  weeks  or  better  at 
eighteen  or  nineteen  days.     A  later  embryo  which  measured  2  5  mm . 


io6 


AGE    OF   EMBRYO    AT    DIFFERENT    STAGES 


crown-rump  measurement,  was  the  result  of  a  coition  that  took 
place  fifty-six  days  before  the  abortion,  so  that  the  embryo  may 
be  regarded  as  having  been  a  httle  less  than  eight  weeks  old. 
These  and  three  other  similar  cases  may  be  shown  in  a  table  thus : 


Length  of  Emb. 

Menstruation 

Copulation 

Probable  Fertiliza- 

Authority 

in  mm. 

Age  in  Days 

Age  in  Days 

tion  Age  in  Days 

About  o.is 

38 

16 

14 

Bryce-Teacher 

1.3 

34 

21 

19 

Eternod 

V.  B.    8.8 

42 

38 

36 

Tandler 

V.  B.  14.0 

65 

47 

45 

Rabl 

V.  B.  18.0 

54 

47 

45 

Mall 

V.  B.  25.0 

75 

56 

54 

Mall 

In  the  fourth  column  two  days  have  been  taken  from  the 
copulation  age  to  estimate  the  fertilization  age.  This  may  be 
too  much  in  some  cases  and  too  little  in  others  and  either  this  or  a 
difference  in  the  rate  of  growth  may  account  for  the  fact  that  two 
embryos  differing  in  their  vertex-breech  measurements  by  4.0 
mm.  appear  to  be  of  the  same  age.  The  14.0  mm.  embryo,  how- 
ever, might  be  assigned  a  fertilization  age  of  44  days  and  the  18.0 
mm.  one  an  age  of  46  days  without  any  violation  of  the  data. 
It  is  interesting  to  note  the  wide  variation  that  obtains  between 
the  menstruation  and  copulation  ages,  the  difference  in  one  case 
being  as  little  as  four  days  and  in  another  as  much  as  twenty-two, 
though  the  average  difference  as  determined  from  statistics  of 
full-term  births  is  about  eleven  days. 

Making  all  possible  corrections  the  exact  age  cannot  be  de- 
termined within  less  than  two  or  three  days  (Mall) ,  but  in  general 
one  may  say  that  embryos  of  2.0  to  3.0  mm.  may  be  assigned  to 
the  fourth  week  of  development,  those  of  5.0  to  6.0  vertex-breech 
length  to  the  latter  part  of  the  fifth  week,  those  of  10. o  mm.  to  the 
end  of  the  sixth  week  and  those  of  25.0  to  28.0  mm.  which  are 
just  passing  into  the  fetus  stage,  to  the  end  ot  the  eighth  week. 
As  regards  the  later  periods  of  development,  the  limits  of  error 
for  any  date  become  of  less  importance.  Schroder  gives  the 
following  measurements  as  the  average : 


LITERATURE  I07 

3d  lunar  month 70-90  mm. 

4th  lunar  month 100-170  mm. 

5th  lunar  month 180-270  mm. 

6th  lunar  month 280-340  mm.        _ 

7th  lunar  month 35«>~38o  mm. 

8th  lunar  month 425  mm. 

Qth  lunar  month 467  mm. 

loth  lunar  month 490-500  mm. 

The  data  concerning  the  weight  of  embryos  of  different  ages 
are  as  yet  very  insufficient,  and  it  is  well  known  that  the  weights 
of  new-born  children  may  vary  greatly,  the  authenticated  ex- 
tremes being,  according  to  Vierordt,  717  grams  and  6123  grams. 
It  is  probable  that  considerable  variations  in  weight  occur  also 
during  fetal  life.  So  far  as  embryos  of  the  first  two  months  are 
concerned,  the  data  are  too  imperfect  for  tabulation;  for  later 
periods  Fehling  gives  the  following  as  average  weights : 

3d  month 20  grams. 

4th  month 120  grams. 

5th  month 285  grams. 

6th  month 635  grams. 

7th  month 1220  grams. 

8th  month 1700  grams. 

9th  month 2240  grams. 

loth  month 3250  grams. 

and  the  results  obtained  by  Jackson  are  essentially  similar. 

LITERATURE 

In  addition  to  the  papers  of  Bryce  and  Teacher,  Eternod,  Fetzer,  Frassi,  Herzog, 

Peters,  Von  Spee,  Strahl  and  Beneke  and  Grosser,  cited  in  the  preceding  chapter,  the 

following  may  be  mentioned: 

J.  L.  Bremer:  "Description  of  a  4  mm.  Human  Embryo,"  Amer.  Journ.  Anat., 
V,  1906. 

J.  Broman:  "Beobachtung  eines  menschlichen  Embryos  von  beinahe  3  mm.  Lange 
mit  specieller  Bemerkung  Uber  die  bei  demselben  befindlichen  Hirnfalten," 
Morpholog.  Arheiten,  v,  1895. 

A.  J.  P.  VAN  DEN  Broek:  "Zur  Kasuistik  junger  menschlicher  Embryonen,"  Anat. 
Hefie,  XLJY,  191 1. 

J._M.  Coste:  ''Histoire  g^nerale  et  particuliere  du  developpement  des  corps  organ- 
ises," Paris,  1 847-1 859. 

W.  E.  Dandy:  "A  Human  Embryo  with  Seven  Pairs  of  Somites,  Measuring  about 
2  mm.  in  Length,"  Amer.  J&iirn.  Anat.,x,  1910. 


Io8  LITERATURE 

A.  Ecker:  "Beitrage  zur  Kenntniss  der  ausserer  Formen  jiingster  menschlichen 

Embryonen,"  Archiv.  fur  Anat.  und  Physiol.,  Anat.  Abth.,  1880. 
C.   Elze:  "  Beschreibung  eines  menschlichen  Embryos  von  zirka  7  mm.  grosster 

Lange,"  Anat.  Hefie,  xxxv,  1907. 
C.  GiACOMiNi:  **Un  oeuf  humain  de  11  jours,"  Archives  Ital.  de  Biologie,  xxxrx,  1898. 
O.   Grosser:  "Altersbestimmung  junges  menschliche  Embryonen — Ovulations — 

und  Menstruationstermin,"  Anat.  Anzeiger,  xlvii,  1914. 
V.   Hensen:  "Beitrag  zur  Morphologic  der  Korperform  und  des   Gehirns   des 

menschlichen  Embryos,"  Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1877. 
W.  His:  "  Anatomie  menschlicher  Embryonen,"  Leipzig,  1880. 

F.  Hochstetter:  "Bilder  der  ausseren  Korperform  einiger  menschlicher  Embryo- 
nen aus  den  beiden  Ersten  Monaten  der  Entwicklung,"  Munich,  1907. 
N.  W.  Ingalls:  "  Beschreibung  eines  menschlichen  Embryos  von  4,9  mm.,"  Arch. 

fiir  mikr.  Anat.,  lxx,  1907. 
C.  M.  Jackson:  "On  the  Prenatal  Growth  of  the  Human  Body  and  the  Relative 

Growth  of  the  Various  Organs  and  Parts,"  Amer,  Journ.  Anat.,  ix,  1909. 
J.  Janosik:  "Zwei  junge  menschliche  Embryonen,"  Archiv  fiir  mikrosk.  Anat.,  xxx, 

1887. 
H.  E.  Jordan:  "Description  of  a  5  mm.  Human  Embryo,"  Anat.  Record,  iii,  1909. 
P.  Jung:  "Beitrage  zur  friihesten  Ei-einbettung  beim  menschlichen  Weibe,"  Berlin, 

1908. 
F.  Keibel:  "Ein  sehr  junges  menschliches  Ei,"  Archiv  fiir  Anat.  und  Physiol.,  Anat. 

Abth.,  1890. 
F.  Keibel:  "Ueber  einen  menschlichen  Embryo  von  6.8  mm.  grosster  Lange," 

Verhandl.  Anatom.  Gesellsch.,  xiii,  1899. 
F.  Keibel  and  C.  Elze:  "Normentafeln  zur  Entwicklungsgeschichte  der  Wirbel- 

tiere,"  Heft  viii,  1908. 
J.  Kollmann:  "Die  Korperform  menschlicher  normaler  und  pathologischer  Em- 
bryonen," Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  Supplement,  1889. 
A.  Low:  "Description  of  a  Human  Embryo  of  13-14  Mesodermic  Somites,"  Journ. 

Anat.  and  Phys.,  xlii,  1908. 
F.  P.  Mall:  "A  Human  Embryo  Twenty-six  Days  Old,"  Journ.  of  Morphology,  v, 

1891. 
F.  P.  Mall:  "A  Human  Embryo  of  the  Second  Week,"  Anat.  Anzeiger^  viii,  1893. 
F.  P.  Mall:  "  Early  Human  Embryos  and  the  Mode  of  their  Preservation,"  Bulletin 

of  the  Johns  Hopkins  Hospital,  iv,  1894. 
F.  P.  Mall:  "On  the  Age  of  Human  Embryos,"  Amer.  Journ.  Anat.,  xxiii,  1918. 
C.  S.  Minot:  "Human  Embryology,"  New  York,  1892. 
J.  MtJLLER:  " Zerglierderungen  menschlicher  Embryonen  aus  friiherer  2^it,"  Archiv 

fiir  Anat.  und  Physiol.,  1830. 
C.  PmsALix:  "Etude  d'un  Embryon  humain  de  ii  millimeters,"  Archives  de  zoolog. 

experimentale  et  generate,  S6r.  2,  VI,  1888. 
H.  Piper:  "Ein  menschlicher  Embryo  von  6.8  mm.  Nackenlinie,"  Archiv  fiir  Anat. 

und  Physiol.,  Anal.  Abth.,  1898. 
C.  Rabl:  "Die  Entwicklung  des  Gesichtes,  Heft   i.  Das  Gesicht  der  Saugetiere, 

Leipzig,  1902. 


LITERATURE  IO9 

G.  Retzius:  "Zur  Kenntniss  der  Entwicklung  der  Korperformen  des  Menschen 
wahrend  der  fotalen  Lebensstufen,"  Biolog.  Untersuch.,  xi,  1904. 

J.  Tandler:  "Ueber  einen  menschlichen  Embryo  von  38  Tage,"  AnaL  Anzeiger, 
XXXI,  1907. 

Allen  Thompson:  "Contributions  to  the  History  of  the  Structure  of  the  Human 
Ovum  and  Embryo  before  the  Third  Week  after  Conception,  with  a  Description 
of  Some  Early  Ova,"  Edinburgh  Med.  and  Surg.  Journal,  iii,  1839.  (See  also 
Froriep's  Neue  Notizen,  xiii,  1840). 

P.  Thompson:  Description  of  a  human  embryo  of  twenty-three  paired  somites," 
Journ.  Anat.  and  Phys.,XLi,  1907. 

F.  W.  Thyng:  "The  Anatomy  of  a  17.8  mm.  human  embryo,"  Amer.  Journ.  Anat., 
XVII,  1914. 

H.  Triepel:  "Altersbestimmung  bei  menschlichen  Embryonen,"  Anal.  Anz.,  XLVI, 
1914. 

H.  Triepel:  "Alter  menschlicher  Embryonen  und  Ovulationstermin,"  Anal.  Anzei- 
ger, XLViii,  1915. 

I.  E.  Wallin:  "A  Human  Embryo  of  Thirteen  Somites,"  Amer.  Journ.  AnaL,  xv, 

1913- 
J.  C.  Watt:     "Description  of  two  young  twin  human  embryos  with  17-19  paired 
somites,"  Puh.  Carnegie  Inst.  No.  222,  Contr.  to  Embryol,  No.  2,  1915. 


CHAPTER  V 

THE  YOLK-STALK,  BELLY-STALK,  AND  FETAL 
MEMBRANES 

The  conditions  to  which  the  embryos  and  larvae  of  the  majority 
of  animals  must  adapt  themselves  are  so  different  from  those  under 
which  the  adult  organisms  exist  that  in  the  early  stages  of  de- 
velopment special  organs  are  very  frequently  developed  which  are 
of  use  only  during  the  embryonic  or  larval  period  and  are  dis- 
carded when  more  advanced  stages  of  development  have  been 
reached.  This  remark  applies  with  especial  force  to  the  human 
embryo  which  leads  for  a  period  of  nine  months  what  may  be 
termed  a  parasitic  existence,  drawing  its  nutrition  from  and 
yielding  up  its  waste  products  to  the  blood  of  the  parent.  In 
order  that  this  may  be  accomplished  certain  special  organs  are 
developed  by  the  embryo,  by  means  of  which  it  forms  an  intimate 
connection  with  the  walls  of  the  uterus,  which,  on  its  part,  be- 
comes greatly  modified,  the  combination  of  embryonic  and  ma- 
ternal structures  producing  what  are  termed  the  deciducB,  owing 
to  their  being  discarded  when  at  birth  the  parasitic  mode  of  life 
is  given  up. 

Furthermore,  it  has  already  been  seen  that  many  peculiar 
modifications  of  development  in  the  human  embryo  result  from 
the  inheritance  of  structures  from  more  or  less  remote  ancestors, 
and  among  the  embryonic  adnexes  are  found  structures  which 
represent  in  a  more  or  less  modified  condition  organs  of  con- 
siderable functional  importance  in  lower  forms.  Such  structures 
are  the  yolk-stalk  and  vesicle,  the  amnion,  and  the  allantois,  and 
for  their  proper  understanding  it  will  be  well  to  consider  briefly 
their  development  in  some  lower  form,  such  as  the  chick. 

At  the  time  when  the  embryo  of  the  chick  begins  to  be  con- 
stricted off  from  the  surface  of  the  large  yolk-mass,  a  fold,  con- 

IIO 


YOLK-STALK    AND    FETAL    MEMBRANES 


III 


sisting  of  ectoderm  and  somatic  mesoderm,  arises  just  outside 
the  embryonic  area,  which  it  completely  surrounds.  As  develop- 
ment proceeds  the  fold  becomes  higher  and  its  edges  gradually 
draw  nearer  together  over  the  dorsal  surface  of  the  embryo  (Fig. 
65,  A,  Af),  and  finally  meet  and  fuse  (Fig.  65,  B  and  C),  so  that 
the  embryo  becomes  enclosed  within  a  sac,  which  is  termed  the 
amnion  and  is  formed  by  the  fusion  of  the  layers  which  consti- 
tuted the  inner  wall  of  the  fold.  The  layers  of  the  outer  wall  of 
the  fold  after  fusion  form  part  of  the  general  ectoderm  and  somatic 


Fig.  65. — Diagrams  Illustrating  the  Formation  of  the  Amnion  and  Allantois 

IN  THE  Chick. 
Af,  Amnion  folds;  Al,  allantois;  Am,  amniotic  cavity;  Ds,  yolk-sac. — (Gegenbaur.) 

mesoderm  which  make  up  the  outer  wall  of  the  ovum  and  together 
are  known  as  the  serosa,  corresponding  to  the  chorion  of  the 
mammalian  embryo.  The  space  which  occurs  between  the  am- 
nion and  the  serosa  is  a  portion  of  the  extra-embryonic  coelom, 
and  is  continuous  with  the  embryonic  pleuroperitoneal  cavity. 
In  the  ovum  of  the  chick,  as  in  that  of  the  reptile,  the  proto- 
plasmic material  is  limited  to  one  pole  and  rests  upon  the  large 
yolk-mass.  As  development  proceeds  the  germ  layers  gradu- 
ally extend  around  the  yolk-mass  and  eventually  completely  en- 


112  THE    AMNION 

close  it,  the  yolk-mass  coming  to  lie  within  the  endodermal  layer, 
which,  together  with  the  splanchnic  mesoderm  which  lines  it, 
forms  what  is  termed  the  yolk-sac.  As  the  embryo  separates  from 
the  yolk-mass  the  yolk-sac  is  constricted  in  its  proximal  portion 
and  so  differentiated  into  a  yolk-stalk  and  a  yolk-sac,  the  contents 
of  the  latter  being  gradually  absorbed  by  the  embryo  during  its 
growth,  its  walls  and  those  of  the  stalk  being  converted  into  a 
portion  of  the  embryonic  digestive  tract. 

In  the  meantime,  however,  from  the  posterior  portion  of  the 
digestive  tract,  behind  the  point  of  attachment  of  the  yolk-sac,  a 
diverticulum  has  begun  to  form  (Fig.  65,  A,  Al).  This  increases 
in  size,  projecting  into  the  extra-embryonic  portion  of  the  pleuro- 
peritoneal  cavity  and  pushing  before  it  the  splanchnic  mesoderm 
which  lines  the  endoderm  (Fig.  65,  B  and  C).  This  is  the  allan- 
tois,  which,  reaching  a  very  considerable  size  in  the  chick  and 
applying  itself  closely  to  the  inside  of  the  serosa,  serves  as  a  respi- 
ratory and  excretory  organ  for  the  embryo,  for  which  purpose  its 
walls  are  richly  supplied  with  blood-vessels,  the  allantoic  arteries 
and  veins. 

Toward  the  end  of  the  incubation  period  both  the  amnion  and 
allantois  begin  to  undergo  retrogressive  changes,  and  just  before 
the  hatching  of  the  young  chick  they  become  completely  dried  up 
and  closely  adherent  to  the  egg-shell,  at  the  same  time  separating 
from  their  point  of  attachment  to  the  body  of  the  young  chick,  so 
that  when  the  chick  leaves  the  egg-shell  it  bursts  through  the 
dried-up  membranes  and  leaves  them  behind  as  useless  structures. 

The  Amnion. — Turning  now  to  the  human  embryo,  it  will  be 
found  that  the  same  organs  are  present,  though  somewhat  modified 
either  in  the  mode  or  the  extent  of  their  development.  A  well 
developed  amnion  occurs,  arising,  however,  in  a  very  different 
manner  from  what  it  does  in  the  chick;  a  large  yolk-sac  occurs  even 
though  it  contains  no  yolk;  and  an  allantois  which  has  no  respira- 
tory or  excretory  functions  is  present,  though  in  a  somewhat 
degenerated  condition.  It  has  been  seen  from  the  description  of 
the  earliest  stages  of  development  that  the  processes  which  occur 
in  the  lower  forms  are  greatly  abbreviated  in  the  human  embryo. 


THE   AMNION  II3 

The  enveloping  layer,  instead  of  gradually  extending  from  one 
pole  to  enclose  the  entire  ovum,  develops  in  situ  during  the  stages 
immediately  succeeding  segmentation,  and  the  extra-embryonic_ 
mesoderm,  instead  of  growing  out  from  the  embryo  to  enclose  the 
yolk-sac,  apparently  also  undergoes  a  precocious  development 
in  situ.  The  earliest  stages  in  the  development  of  the  amnion 
are  not  yet  known  for  the  human  embryo,  but  from  the  condition 
in  which  it  is  found  in  the  Peters  embryo  (Fig.  38)  and  in  the 
embryo  v.H.  of  von  Spec  (Fig.  40)  it  is  probable  that  it  arises, 
not  by  the  fusion  of  the  edges  of  a  fold,  as  in  the  chick,  but  by  a 
vacuolization  of  a  portion  of  the  inner  cell-mass,  as  has  been  de- 
scribed as  occurring  in  the  bat  (p.  57).  It  is,  then,  a  closed  cavity 
from  the  very  beginning,  the  floor  of  the  cavity  being  formed  by 
the  embryonic  disk,  its  posterior  wall  by  the  anterior  surface  of 
the  belly-stalk,  while  its  roof  and  sides  are  thin  and  composed  of  a 
single  layer  of  flattened  ectodermal  cells  lined  on  the  outside 
by  a  layer  of  mesoderm  continuous  with  the  somatic  mesoderm 
of  the  embryo  and  the  mesoderm  of  the  belly-stalk  (Fig.  66,  A). 

When  the  bending  downward  of  the  peripheral  portions  of  the 
embryonic  disk  to  close  in  the  ventral  surface  of  the  embryo  oc- 
curs, the  line  of  attachment  of  the  amnion  to  the  disk  is  also 
carried  ventrally  (Fig.  66,  B),  so  that  when  the  constriction  off 
of  the  embryo  is  practically  completed,  the  amnion  is  attached 
anteriorly  to  the  margin  of  the  umbilicus  and  posteriorly  to  the 
extremity  of  the  band  of  ectoderm  lining  what  may  now  be  con- 
sidered the  posterior  surface  of  the  belly-stalk,  while  at  the  sides 
it  is  attached  along  an  oblique  line  joining  these  two  points  (Fig. 
66,  B  and  C,  in  which  the  attachment  of  the  amnion  is  indicated 
by  the  broken  line) . 

Leaving  aside  for  the  present  the  changes  which  occur  in  the 
attachment  of  the  amnion  to  the  embryo  (see  p.  119),  it  may  be 
said  that  during  the  later  growth  of  the  embryo  the  amniotic 
cavity  increases  in  size  until  finally  its  walls  come  into  contact 
with  the  chorion,  the  extra-embryonic  body-cavity  being  thus 
practically  obliterated  (Fig.  66,  D),  though  no  actual  fusion  of 
amnion  and  chorion  occurs.     Suspended  by  the  umbilical  cord 


114 


THE   AMNION 


which  has  by  this  time  developed,  the  embryo  floats  freely  in  the 
amniotic  cavity,  which  is  filled  by  a  fluid,  the  liquor  amnii,  whose 
origin  is  involved  in  doubt,  some  authors  maintaining  that  it  in- 
filtrates into  the  cavity  from  the  maternal  tissues,  while  others 
hold  that  a  certain  amount  of  it  at  least  is  derived  from  the  em- 


FiG.  66. — Diagrams  Illustrating  the  Formation  of  the  Umbilical  Cord. 
The  heavy  black  line  represents  the  embryonic  ectoderm;  the  dotted  line  repre- 
sents  the  line  of  reflexion  of  the  body  ectoderm  into  that  of  the  amnion.     Ac, 
Amniotic  cavity;  Al,  allantois;  Be,  extra-embryonic  coelum;  Bs,  belly-stalk;  Ch, 
horion;  P,  placenta;   Uc,  umbilical  chord;   V,  chorionic  villi;   Ys,  yolk-sac. 

bryo.  It  is  a  fluid  with  a  specific  gravity  of  about  1.003  ^^nd  con- 
tains about  I  per  cent,  of  solids,  principally  albumin,  grape-sugar, 
and  urea,  the  last  constituent  probably  coming  from  the  embryo. 
When  present  in  greatest  quantity — that  is  to  say,  at  about  the 
beginning  of  the  last  month  of  pregnancy — it  varies  in  amount 
between  one-half  and  three-fourths  of  a  liter,  but  during  the  last 
month  it  diminishes  to  about  half  that  quantity.     To  protect  the 


THE    YOLK-SAC  II5 

epidermis  of  the  fetus  from  maceration  during  its  prolonged  im- 
mersion in  the  liquor  amnii,  the  sebaceous  glands  of  the  skin  at 
about  the  sixth  month  of  development  pour  out  upon  the  surface 
of  the  body  a  white  fatty  secretion  known  as  the  vernix  caseosa. 

During  parturition  the  amnion,  as  a  rule,  ruptures  as  the  re- 
sult of  the  contraction  of  the  uterine  walls  and  the  liquor  amnii 
escapes  as  the  *' waters,*'  a  phenomenon  which  normally  precedes 
the  delivery  of  the  child.  As  a  rule,  the  rupture  is  sufficiently  ex- 
tensive to  allow  the  passage  of  the  child,  the  amnion  remaining 
behind  in  the  uterus,  to  be  subsequently  expelled  along  with  the 
deciduae. 

Occasionally  it  happens,  however,  that  the  amnion  is  sufficiently 
strong  to  withstand  the  pressure  exerted  upon  it  by  the  uterine  con- 
tractions and  the  child  is  born  still  enveloped  in  the  amnion,  which,  in 
such  cases,  is  popularly  known  as  the  *'caul,"  the  possession  of  which, 
according  to  an  old  superstition,  marks  the  child  as  a  favorite  of 
fortune. 

As  stated  above,  the  liquor  amnii  varies  considerably  in  amount  in 
different  cases,  and  occasionally  it  may  be  present  in  excessive  quanti- 
ties, producing  a  condition  known  as  hydramnios.  On  the  other  hand, 
the  amount  may  fall  considerably  below  the  normal,  in  which  case  the 
amnion  may  form  abnormal  unions  with  the  embryo,  sometimes  pro- 
ducing malformations.  Occasionally  also  bands  of  a  fibrous  character 
traverse  the  amniotic  cavity  and,  tightening  upon  the  embryo  during 
its  growth,  may  produce  various  malformations,  such  as  scars,  sphtting 
of  the  eye  lids  or  lips,  or  even  amputation  of  a  limb. 

The  Yolk-sac. — The  probable  mode  of  development  of  the 
yolk-sac  in  the  human  embryo,  and  its  differentiation  into  yolk- 
stalk  and  yolk-vesicle  have  already  been  described  (p.  89).  When 
these  changes  have  been  completed,  the  vesicle  is  a  small  pyriform 
structure  lying  between  the  amnion  and  the  chorionic  mesoderm, 
some  distance  away  from  the  extremity  of  the  umbilical  cord  (Fig. 
66,  D),  and  the  stalk  is  a  long  slender  column  of  cells  extending 
from  the  vesicle  through  the  umbilical  cord  to  unite  with  the  in- 
testinal tract  of  the  embryo.  The  vesicle  persists  until  birth  and 
may  be  found  among  the  decidual  tissues  as  a  small  sac  measuring 
from  3  to  10  mm.  in  its  longest  diameter.  The  stalk,  however, 
early  undergoes  degeneration,  the  lumen  which  it  at  first  contains 


Il6  THE    ALLANTOIS    AND   BELLY-STALK 

becoming  obliterated  and  its  endoderm  also  disappearing  as  early 
as  the  end  of  the  second  month  of  development.  The  portion  of 
the  stalk  which  extends  from  the  umbilicus  to  the  intestine  usually 
shares  in  the  degeneration  and  disappears,  but  in  about  3  per  cent, 
of  cases  it  persists,  forming  a  more  or  less  extensive  diverticulum 
of  the  lower  part  of  the  small  intestine,  sometimes  only  half  an 
inch  or  so  in  length  and  sometimes  much  larger.  It  may  or  may 
not  retain  connection  with  the  abdominal  wall  at  the  umbilicus, 
and  is  known  as  MeckeVs  diverticulum. 

This  embryonic  rudiment  is  of  no  Httle  importance,  since,  when 
present,  it  is  apt  to  undergo  invagination  into  the  lumen  of  the  small 
intestine  and  so  occlude  it.  How  frequently  this  happens  relatively 
to  the  occurrence  of  the  diverticulum  may  be  judged  from  the  fact 
that  out  of  one  hundred  cases  of  occlusion  of  the  small  intestine  six 
were  due  to  an  invagination  of  the  diverticulum. 

In  the  reptiles  and  birds  the  yolk-sac  is  abundantly  supplied 
with  blood-vessels  by  means  of  which  the  absorption  of  the  yolk  is 
carried  on,  and  even  although  the  functional  importance  of  the 
yolk-sac  as  an  organ  of  nutrition  is  almost  nil  in  the  human 
embryo,  yet  it  still  retains  a  well-developed  blood-supply,  the 
walls  of  the  vesicle,  especially,  possessing  a  rich  network  of  vessels. 
The  future  history  of  these  vessels,  which  are  known  as  the 
vitelline  vessels,  will  be  described  later  on. 

The  Allantois  and  Belly-stalk. — It  has  been  seen  that  in 
reptilian  and  avian  embryos  the  allantois  reaches  a  high  degree  of 
development  and  functions  as  a  respiratory  and  excretory  organ 
by  coming  into  contact  with  what  is  comparable  to  the  chorion  of 
the  mammalian  embryo.  In  man  it  is  very  much  modified  both 
in  its  mode  of  development  and  in  its  relations  to  other  parts,  so 
that  its  resemblance  to  the  avian  organ  is  somewhat  obscured. 
The  differences  depend  partly  upon  the  remarkable  abbreviation 
manifested  in  the  early  development  of  the  human  embryo  and 
partly  upon  the  fact  that  the  allantois  serves  to  place  the  embryo 
in  relation  with  the  maternal  blood,  instead  of  with  the  external 
atmosphere,  as  is  the  case  in  the  egg-laying  forms.     Thus,  the 


THE    ALLANTOTS    AND  BELLY-STALK  TI7 

endodermal  portion  of  the  allantois,  instead  of  arising  from  the 
intestine  and  pushing  before  it  a  layer  of  splanchnic  mesoderm 
to  form  a  large  sac  lying  freely  in  the  extra-embryonic  portion  of 
the  body-cavity,  appears  in  the  human  embryo  before  the  intes-' 
tine  has  differentiated  from  the  yolk-sac  and  pushes  its  way  into 
the  solid  mass  of  mesoderm  which  forms  the  belly-stalk  (Fig. 
66,  A).  To  understand  the  significance  of  this  process  it  is  neces- 
sary to  recall  the  abbreviation  in  the  human  embryo  of  the  de- 
velopment of  the  extra-embryonic  mesoderm  and  body-cavity. 
Instead  of  growing  out  from  the  embryonic  area,  as  it  does  in 
the  lower  forms,  this  mesoderm  develops  in  situ  from  the  cellular 
magma  and,  furthermore,  the  extra- 
embryonic body-cavity  arises  before 
there  is  any  trace  of  a  splitting  of  the 
embryonic  mesoderm  (Fig.  39).  The 
belly-stalk,  whose  development  from  a 
portion  of  the  inner  cell-mass  has 
already  been  traced  (p.  72),  is  to  be 
regarded  as  a  portion  of  the  body  of  "---^/i 

the  embryo,  since  the  ectoderm  which        ^^^-  67.— Transverse  Sec- 

•^      '  ^  TION       THROUGH      THE      BeLLY- 

covers  one  surface  of  it  resembles  ex-  stalk  of  an  Embryo  of  2.1s 
actly  that  of  the  embryonic   disk  and    ^^\       tt   v-m-    ,    ,  „ 

•^  ,  -^  Aa,     Umbilical     (allantoic) 

shows    an    extension    backward    of    the     artery;  ah,  allantois;  am,  am- 

1    11  •,  r  /T7«  nion;   Va,  umbilical    (allantoic) 

medullary  groove  upon  its  surface  (Fig.    ^ein— (His.) 
67).     The  mesoderm,  therefore,  of  the 

belly-stalk  is  to  he  regarded  as  a  portion  of  the  embryonic 
mesoderm  which  has  not  yet  undergone  a  splitting  into  somatic 
and  splanchnic  layers,  and,  indeed,  it  never  does  undergo  such  a 
spHtting,  so  that  there  is  no  body-cavity  into  which  the  endo- 
dermal allantoic  diverticulum  can  grow. 

But  this  does  not  account  for  all  the  peculiarities  of  the  human 
allantois.  In  the  birds,  and  indeed  in  the  lower  oviparous  mam- 
mals, the  endodermal  portion  of  the  allantois  is  equally  developed 
with  the  mesodermal  portion,  the  allantois  being  an  extensive  sac 
whose  cavity  is  filled  with  fluid,  and  this  is  also  true  of  such  mam- 
mals as  the  marsupials,  the  rabbit,  and  the  ruminants.     In  man, 


Il8  THE    ALLANIOIS    AND   BELLY-STALK 

however,  the  endodermal  diverticulum  never  becomes  a  sac-like 
structure,  but  is  a  slender  tube  extending  from  the  intestine  to  the 
chorion  and  lying  in  the  substance  of  the  mesoderm  of  the  belly- 
stalk  (Fig.  66,  D),  the  greater  portion  of  which  is  to  be  regarded  as 
homologous  with  the  relatively  thin  layer  of  splanchnic  mesoderm 
covering  the  endodermal  diverticulum  of  the  chick.  An  explana- 
tion of  this  disparity  in  the  development  of  the  mesodermal  and 
endodermal  portions  of  the  human  allantois  is  perhaps  to  be 
found  in  the  altered  conditions  under  which  the  respiration  and 
secretion  take  place.  In  all  forms,  the  lower  as  well  as  the  higher, 
it  is  the  mesoderm  which  is  the  more  important  constituent  of 
the  allantois,  since  in  it  the  blood-vessels,  upon  whose  presence 
the  physiological  functions  depend,  arise  and  are  embedded. 
In  the  birds  and  oviparous  mammals  there  are  no  means  by  which 
excreted  material  can  be  passed  to  the  exterior  of  the  ovum,  and 
it  is,  therefore,  stored  up  within  the  cavity  of  the  allantois,  the 
allantoic  fluid  containing  considerable  quantities  of  nitrogen,  indi- 
cating the  presence  of  urea.  In  the  higher  mammals  the  intimate 
relations  which  develop  between  the  chorion  and  the  uterine  walls 
allow  of  the  passage  of  excreted  fluids  into  the  maternal  blood ;  ^nd 
the  more  intimate  these  relations,  the  less  necessity  there  is  for 
an  allantoic  cavity  in  which  excreted  fluid  may  be  stored  up.  The 
difference  in  the  development  of  the  cavity  in  the  ruminants,  for 
example,  and  man  depends  probably  upon  the  greater  intimacy 
of  the  union  between  ovum  and  uterus  in  the  latter,  the  arrange- 
ment for  the  passage  of  the  excreted  material  into  the  maternal 
blood  being  so  perfect  that  there  is  practically  no  need  for  the 
development  of  an  allantoic  cavity. 

The  portion  of  the  endodermal  diverticulum  which  is  enclosed 
within  the  umbilical  cord  persists  until  birth  in  a  more  or  less 
rudimentary  condition,  but  the  intra-embryonic  portion  extending 
from  the  apex  of  the  bladder  to  the  umbilicus  becomes  converted 
into  a  solid  cord  of  fibrous  tissue  termed  the  urachus. 

Occasionally  a  lumen  persists  in  the  urachal  portion  of  the  allantois 
and  may  open  to  the  exterior  at  the  umbilicus,  in  which  case  urine  from 
the  bladder  may  escape  at  the  umbilicus. 


THE   UMBILICAL   CORD  II 9 

•  Since  the  allantois  in  the  human  embryo,  as  well  as  in  the 
lower.forms,  is  responsible  for  respiration  and  excretion,  its  blood- 
vessels are  well  developed.  They  are  represented  in  the  belly- 
stalk  by  two  veins  and  two  arteries  (Fig.  67),  known  in  human 
embryology  as  the  umbilical  veins  and  arteries.  These  extend 
from  the  body  of  the  embryo  out  to  the  chorion,  there  branching 
repeatedly  to  enter  the  numerous  chorionic  villi  by  which  the 
embryonic  tissues  are  placed  in  relation  with  the  maternal. 

The  Umbilical  Cord. — During  the  process  of  closing  in  of  the 
ventral  surface  of  the  embryo  a  stage  is  reached  in  which  the 
embryonic  and  extra-embryonic  portions  of  the  body-cavity  are 
completely  separated  except  for  a  small  area,  the  umbilicus 
through  which  the  yolk-stalk  passes  out  (Fig.  66,  B) .  At  the  edges 
of  this  area  in  front  and  at  the  sides  the  embryomc  ectoderm  and 
somatic  mesoderm  become  continuous  with  the  corresponding 
layers  of  the  amnion,  but  posteriorly  the  line  of  attachment  of  the 
amnion  passes  up  upon  the  sides  of  the  belly-stalk  (Fig.  66,  B), 
so  that  the  whole  of  the  ventral  surface  of  the  stalk  is  entirely  un- 
covered by  ectoderm,  this  layer  being  limited  to  its  dorsal  surface 
(Fig.  67).  In  subsequent  stages  the  embryonic  ectoderm  and 
somatic  mesoderm  at  the  edges  of  the  umbilicus  grow  out  ventrally , 
carrying  with  them  the  line  of  attachment  of  the  amnion  and 
forming  a  tube  which  encloses  the  proximal  part  of  the  yolk- 
stalk.  The  ectoderm  of  the  belly-stalk  at  the  same  time  extend- 
ing more  laterally,  the  condition  represented  in  Fig.  66,  C,  is 
produced,  and,  these  processes  continuing,  the  entire  belly-stalk, 
together  with  the  yolk-stalk,  becomes  enclosed  within  a  cylindrical 
cord  extending  from  the  ventral  surface  of  the  body  to  the  chorion 
and  forming  the  umbilical  cord  (Fig.  66,  D). 

From  this  mode  of  development  it  is  evident  that  the  cord  is, 
strictly  speaking,  a  portion  of  the  embryo,  its  surfaces  being 
completely  covered  by  embryonic  ectoderm,  the  amnion  being 
carried  during  its  formation  further  and  further  from  the  umbilicus 
until  finally  it  is  attached  around  the  distal  extremity  of  the  cord. 

In  enclosing  the  yolk-stalk  the  umbilical  cord  encloses  also  a 
small  portion  of  what  was  originally  the  extra-embryonic  body 


I20 


THE    CHORION 


al  -r 


uv 


ua 


uv 


Fig.  68. — Transverse  Sections  of  the  Umbilical   Cord  of  Embryos  of  (.4) 

1.8  CM.    AND    {B)    25    CM. 

al,  Allantois;  c,  ccBlom:  ua,  umbilical  artery;  uv,  umbilical  vein;   ys,  yolk-stalk. 


THE   CHORION  l2I 

cavity  surrounding  the  yolk-stalk.  A  section  of  the  cord  in  an 
early  stage  of  its  development  (Fig.  68,  A)  will  show  a  thick 
mass  of  mesoderm  occupying  its  dorsal  region;  this  represents 
the  mesoderm  of  the  belly-stalk  and  contains  the  allantois  and  the 
umbilical  arteries  and  vein  (the  two  veins  originally  present  in 
the  belly-stalk  having  fused),  while  toward  the  ventral  surface 
there  will  be  seen  a  distinct  cavity  in  which  lies  the  yolk  stalk 
with  its  accompanying  blood-vessels.  The  .portion  of  this 
coelom  nearest  the  body  of  the  embryo  becomes  much  enlarged, 
and  during  the  second  month  of  development  contains  some  coils 
of  the  small  intestine,  but  later  the  entire  cavity  becomes  more 
and  more  encroached  upon  by  the  growth  of  the  mesoderm,  and 
at  about  the  fourth  month  is  entirely  obliterated.  A  section  of 
the  cord  subsequent  to  that  period  of  development  will  show  a 
solid  mass  of  mesoderm  in  which  are  embedded  the  umbilical  ar- 
teries and  vein,  the  allantois,  and  the  rudiments  of  the  yolk- 
stalk  (Fig.  68,  B). 

When  fully  formed,  the  umbilical  cord  measures  on  the  aver- 
age 55  cm.  in  length,  though  it  varies  considerably  in  different 
cases,  and  has  a  diameter  of  about  1.5  cm.  It  presents  the  ap- 
pearance of  being  spirally  twisted,  an  appearance  largely  due, 
however,  to  the  spiral  course  pursued  by  the  umbilical  arteries, 
though  the  entire  cord  may  undergo  a  certain  amount  of  torsion 
from  the  movements  of  the  embryo  in  the  later  stages  of  develop- 
ment and  may  even  be  knotted.  The  greater  part  of  its  sub- 
stance is  formed  by  the  mesoderm,  the  cells  of  which  become 
stellate  and  form  a  reticulum,  the  meshes  of  which  are  occupied 
by  connective-tissue  fibrils  and  a  mucous  fluid  which  gives  to  the 
tissue  a  jelly-like  consistence,  whence  it  has  received  the  name  of 
Wharton^s  jelly. 

The  Chorion. — To  understand  the  developmental  changes 
which  the  chorion  undergoes  it  wn'U  be  of  advantage  to  obtain  some 
insight  into  the  manner  in  which  the  ovum  becomes  implanted  in 
the  wall  of  the  uterus.  Nothing  is  known  as  to  how  this  implanta- 
tion is  effected  in  the  case  of  the  human  ovum ;  it  has  already  been 
accomplished  in  the  youngest  ovum  at  present  known.     But  the 


122 


IHE    CHORION 


process  has  been  observed  in  other  mammals,  and  what  takes 
place  in  Spermophilus,  for  example,  may  be  supposed  to  give  a 
clue  to  what  occurs  in  the  human  ovum.  In  the  spermophile  the 
ovum  lies  free  in  the  uterine  cavity  up  to  a  stage  at  which  the 
vacuolization  of  the  central  cells  is  almost  completed  (Fig.  69, 
A).  At  one  region  of  the  covering  layer  the  cells  become  thicker 
and  later  form  a  syncytial  projection  or  knob  which  comes  into 


Fig.  69. — Successive  Stages  in  the  Implantation  of  the  Ovum  of 
THE  Spermophile. 
a.  Syncytial  knob;  k,  inner  cell-mass. — (Resjek.) 

contact  with  the  uterine  mucosa  (Fig.  69,  B),  and  at  the  point  of 
contact  the  mucosa  cells  undergo  degeneration,  allowing  the  knob 
to  come  into  relation  with  the  deeper  tissues  of  the  uterus  (Fig. 
69,  C),  the  process  apparently  being  one  in  which  the  mucosa  cells 
are  eroded  by  the  syncytial  knob. 

It  seems  probable  that  in  the  human  ovum  the  process  is  at 
first  of  a  similar  nature  and  that  as  the  covering  layer  cells  come 
into  contact  with  the  deeper  layes  of  the  uterus,  these  too  are 
eroded,   and,   the  uterine  blood-vessels   being  included   in   the 


THE    CHORION 


123 


Umy 

S 

^(1 

^^dL 

i 

1 

wM 

I 

Pig.  70. — Diagrams  Illustrating  the  Implantation  of  the  Ovum. 
ac.  Amniotic  cavity;  hs,  belly-stalk;  cf,  chorion  frondosum;  cl,  chorion  laeve;   dc, 
decidua  capsularis;  ic,  inner  cell-mass;  s,  space  surrounding   ovum  which  becomes 
the  intervillous  space;  um,  uterine  mucosa;  v,  chorionic  villus;  ys,  yolk-sac. 


124 


THE    CHORION 


erosion  process,  an  extravasation  of  blood  plasma  and  corpuscles 
occurs  in  the  vicinity  of  the  burrowing  ovum.  In  the  meantime 
the  ovum  has  increased  considerably  in  size,  its  growth  in  these 
early  stages  being  especially  rapid,  and  the  area  of  contact 
consequently  increases  in  size,  entailing  continued  erosion  of  the 
uterine  mucosa.  At  the  same  time,  too,  the  uterine  tissues 
surrounding  the  ovum  grow  up  around  it,  forming  at  first  as  it 


ScL 


^^^r'f^^^- 


f'»'  ..'*.'  ^*. •';#.•"'  '  S  /'     .<    '      \ 


<-i.f^^  ^~  -v.' 


EV. 


Fig.  71. — Section  of  an  Ovum  of  i  mm.     A  Section  of  the  Embryo  Lies  in  the 

Lower  Part  of  the  Cavity  of  the  Ovum. 

D,  Decidua;  R.U .,  uterine  epithelium;  Sch,  blood-clot  closing  the  aperture  left  by 

the  sinking  of  the  ovum  into  the  uterine  mucosa. — {From  Strahl,  after  Peters.) 

were  a  circular  wall  (Fig.  70,  A),  and  eventually  completely 
enclose  it,  forming  an  envelope  known  as  the  decidua  capsularis 
or  reflexa.  The  blood  extravasation  is  now  contained  within  a 
closed  space  bounded  on  the  one  hand  by  the  uterine  tissues 
and  on  the  other  by  the  wall  of  the  ovum  (Fig.  70,  B). 

The  youngest  known  human  ova  have  already  reached  ap- 
proximately this  stage.  Thus,  the  Peters  ovum  (Fig.  71)  had 
already  sunk  deeply  into  the  uterine  mucosa,  the  point  of  entrance 


THE    CHORION  12  5 

being  indicated  by  a  gap  in  the  decidiia  capsularis,  closed  in  this 
case  by  a  patch  of  coagulated  blood  (Sch).  Ihe  uterine  tissues 
in  the  immediate  vicinity  of  the  ovum  were  much  swollen  and 
apparently  somewhat  necrotic  and  their  blood-vessels  could  be 
seen  to  communicate  with  the  space  between  the  wall  of  the 
ovum  and  the  maternal  tissues.  This  space,  however,  was  con- 
verted into  an  irregular  network  of  blood  lacunae  by  anastomosing 
cords  of  cells,  which  arose  from  the  wall  of  the  ovum  and  ex- 
tended through  the  space  to  the  maternal  tissues;  these  cords  of 
cells  are  represented  in  Fig.  71  by  the  darker  masses  projecting 
from  the  wall  of  the  ovum  and  scattered  among  the  paler  blood 
lacunae.  This  stage  of  implantation  of  the  ovum  is  shown  dia- 
grammatically  in  Fig.  70,  B,  where,  for  simplicity's  sake,  the  cell 
cords  are  represented  merely  as  processes  radiating  from  the 
ovum  without  reaching  the  maternal  tissues. 

The  cell  cords  are  derivatives  of  the  trophoblast  and  are,  there 
fore,  of  embryonic  origin.  If  examined  under  a  higher  magnifica- 
tion than  that  shown  in  Fig.  70  they  will  be  seen  to  be  composed 
an  axial  core  of  cells  with  distinct  outlines,  enclosed  within  a  layer 
of  protoplasm  which  lacks  all  traces  of  cell  boundaries,  although  it 
contains  numerous  nuclei,  being  what  is  termed  a  syncytium  or 
Plasmodium.  The  two  tissues  represent  the  two  layers  differen- 
tiated from  the  original  trophoblast,  the  cellular  one  being  the 
cyto-trophoUast  and  the  plasmodial  one  the  plasmodi-trophohlast. 
The  latter  is  the  tissue  that  comes  into  contact  with  the  maternal 
blood  contained  in  the  lacunar  spaces  and  with  the  maternal 
tissues,  in  connection  with  these  latter  sometimes  developing  into 
masses  of  considerable  extent.  To  the  plasmodi-trophoblast 
may  be  ascribed  the  active  part  in  the  destruction  of  the  maternal 
tissues  and  probably  also  the  absorption  of  the  products  of  the 
destruction  for  the  nutrition  of  the  growing  ovum.  For  up  to 
this  stage  the  ovum  has  been  playing  the  role  of  a  parasite  thriving 
upon  the  tissues  of  its  host. 

The  food  material  that  the  ovum  thus  obtains  may  con- 
veniently be  termed  the  emhryotroph  and  the  type  of  placentation 
which  obtains  up  to  this  stage  and  for  some  time  longer  may  be 


126  THE   CHORION 

termed  the  embryotrophic  type.  But  even  in  the  Peters  ovum  the 
preparation  for  another  type  has  begun.  In  earlier  stages  the 
cell  cords  were  entirely  trophoblastic,  but  in  this  ovum  (Fig.  71) 
processes  from  the  chorionic  mesoderm  may  be  seen  projecting 
into  the  bases  of  the  cell  cords,  and  in  later  stages  these  processes 
extend  farther  and  farther  into  the  axis  of  each  cord,  the 
anastomoses  of  the  cords  disappear  and  the  cords  themselves 
become  converted  into  branching  processes,  the  chorionic  villij 
which  project  from  the  entire  surface  of  the  ovum  (Fig.  72)  into  the 


Fig.  72. — Entire   Ovum  Aborted   at  about  the   Beginning   of   the   Second 
Month.      XiM-- — (Grosser.) 

surrounding  space,  and  are  bathed  by  the  maternal  blood  contained 
in  the  surrounding  space,  which  may  now  be  known  as  the  inter- 
villous  space.  Toward  the  maternal  surface  of  the  space  some 
masses  of  the  trophoblast  still  persist,  uniting  the  extremities  of 
certain  of  the  villi  to  the  enclosing  uterine  wall,  such  villi  being 
termed  fixation  villi  to  distinguish  them  from  others,  which  project 
freely  into  the  intervillous  space.  Later,  when  the  embryonic 
blood-vessels  develop,  those  associated  with  the  allantois  extend 
outward  into  the  chorionic  mesoderm  and  thence  send  branches 
into  each  villus.     The  second  type  of  placentation,  the  hcemotro- 


THE   CHORION 


127 


phic  type,  is  thus  established,  the  fetal  blood  contained  in  the 
vessels  of  the  villi  receiving  nutrition  through  the  walls  of  the 
villi  from  the  maternal  blood  contained  in  the  intervillous  space, 
and,  similarly,  transferring  waste  products  to  it. 

At  first,  as  stated  above,  the  villi  usually  cover  the  entire 
surface  of  the  ovum,  but  later,  as  the  ovum  increases  in  size, 
those  villi  which  are  remote  from  the  attachment  of  the  belly-stalk 
to  the  chorion  are  placed  at  a  disadvantage  so  far  as  their  blood 
supply  is  concerned  and  gradually  disappear,  and  this  process 


Fig.  73. — Two  Villi  from  the  Chorion  of  an  Embryo  of  7  mm. 


continues  until,  finally,  only  those  villi  are  retained  which  are  in 
the  immediate  region  of  the  belly-stalk  (Fig.  70,  C),  these  per- 
sisting to  form  the  fetal  portion  of  the  placenta.  By  these  changes 
the  chorion  becomes  differentiated  into  two  regions  (Fig.  70,  C), 
one  of  which  is  destitute  of  villi  and  is  termed  the  chorion  Iceve, 
while  the  other,  provided  with  them,  is  known  as  the  chorion 
frondosum. 

Occasionally  one  or  more  patches  of  villi  may  persist  in  the  area 
that  normally  becomes  the  chorion  laeve  and  thus  accessory  placentce 
{placentcB  succenturiatce) ,  varying  in  number  and  size,  may  be  formed. 


128 


THE   CHORION 


p.^    ..—Transverse  Sections  through  Chorionic  Villi  in   (A)   the  Fifth 

*  AND  (B)  THE  Seventh  Month  of  Development. 

cf    Canalized  fibrin;  Ic,  Langhans  cells;  5.   syncytium  .-(A  ^hich  is   more  highly 
•"  magnified  than  B,  from  Szymonowicz;  B  from  Mtnot.) 


THE   CHORION 


129 


The  villi  when  fully  formed  are  processes  of  the  chorion, 
branching  profusely  and  irregularly  (Fig.  73),  and  each  consists  of~ 
a  core  of  mesoderm,  containing  blood-vessels,  enclosed  within  a 
double  layer  of  trophoblastic  tissue  (Fig.  74,  A).  The  inner 
layer  consists  of  a  sheet  of  well-defined  cells  arranged  in  a  single 
series;  it  is  derived  from  the  cyto-trophoblast  and  forms  what  is 
known  as  the  layer  of  Langhans  cells.  The  outer  layer  is  syncytial 
in  structure  and  is  formed  from  the  plasmodi-trophoblast. 


I  Pig.  75. — Mature  Placenta  after  Separation  from  the  uterus. 
c.    Cotyledons;    ch,    chorion,    amnion,    and   decidua   vera;    um,    umbilical   cord. — 

{Kollmann.) 

As  development  proceeds  the  villi,  which  are  at  first  distributed 
evenly  over  the  chorion  frondosum,  become  separated  into  groups 
termed  cotyledons  (Fig.  75)  by  the  growth  into  the  intervillous 
space  of  trabeculae  from  the  walls  of  the  uterus,  the  fixation  villi 
becoming  connected  with  these  septa  as  well  as  with  the  general 
uterine  wall.  The  ectoderm  of  the  villi  undergoes  also  certain 
changes  with  advancing  growth,  the  layer  of  Langhans  cells 
disappearing  except  in  small  areas  scattered  irregularly  in  the 
villi,  and  the  syncytium,  though  persisting,  undergoes  local  thick- 
enings which  become  replaced,  more  or  less  extensively,  by  de- 
positions of  fibrin  (Fig.  74  B,  cj). 


130 


THE   DECIDU^ 


The  changes  which  occur  during  the  later  stages  of  develop- 
ment in  the  chorion  are  very  similar  to  those  described  for  the 
villi.  Thus,  the  mesoderm  thickens,  its  outermost  layers  be- 
coming exceedingly  fibrillar  in  structure,  while  later,  as  in  the 
villi,  the  syncytial  layer  of  its  trophoblast  is  replaced  in  irregu- 


inas 


rCW^'-^-ftf 


Fig.  76. — Section  through  the  Placental  Chorion  of  an  Embryo  of  Seven 

Months. 
c,  Cell  layer;  ep,  remnants  of  epithelium;  fb,  fibrin  layer;  mes,  mesoderm. — {Minot.) 


lar  patches  by  a  peculiar  form  of  fibrin  which  is  traversed  by 
flattened  anastomosing  spaces  and  to  which  the  name  canalized 
fibrin  or  fibrinoid  has  been  applied  (Fig.  76). 

The  Deciduae. — It  has  been  pointed  out  (p.  27)  that  in  connec- 
tion with  the  phenomenon  of  menstruation  periodic  alterations 


THE   DECIDUiE  I3I 

occur  in  the  mucous  membrane  of  the  uterus.  If  during  one  of 
these  periods  a  fertilized  ovum  reaches  the  uterus,  the  desquama- 
tion of  portions  of  the  epitheHum  does  not  occur  nor  is  there  any 
appreciable  hemorrhage  into  the  cavity  of  the  uterus;  the  uterine 
mucosa  remains  in  what  is  practically  the  ante-menstrual  condi- 
tion until  the  conclusion  of  pregnancy,  when,  after  the  birth  of 
the  fetus,  a  considerable  portion  of  its  thickness  is  expelled  from 


Fig.  77. — Diagram  showing  the  relations  of  the  Fetal  Membranes. 

Am,  Amnion;  Ch,  chorion;  M,  muscular  wall  of  uterus;  C,  decidua  capsularis;  B, 

decidua  basalis;  V,  decidua  vera;  Y,  yolk-stall:. 

the  uterus,  forming  what  is  termed  the  deciduce.  In  other  words, 
the  sloughing  of  the  uterine  tissue  which  concludes  the  process  of 
menstruation  is  postponed  until  the  close  of  pregnancy,  and  then 
takes  place  simultaneously  over  the  whole  extent  of  the  uterus. 
Of  course,  the  changes  in  the  uterine  tissues  are  somewhat  more 
extensive  during  pregnancy  than  during  menstruation,  but  there 
is  an  undoubted  fundamental  similarity  in  the  changes  during  the 
two  processes. 


132 


THE   DECIDU^ 


The  human  ovum  comes  into  direct  apposition  with  only  a 
small  portion  of  the  uterine  wall,  and  the  changes  which  this 
portion  of  the  wall  undergoes  differ  somewhat  from  those  occur- 
ring elsewhere.  Consequently  it  becomes  possible  to  divide  the 
deciduae  into  (i)  a  portion  which  is  not  in  direct  contact  with  the 
ovum,  the  decidua  vera  (Fig.  77,  V)  and  (2)  a  portion  which  is. 
The  latter  portion  is  again  capable  of  division.     The  ovum  be- 


FiG.  78. — Surface  view  of  Half  of  the  Decidua  Vera  at  the  End  of  the 

Third  Week  of  Gestation. 

d.  Mucous  membrane  of  the  Fallopian  tubes;  ds,  prolongation  of  the  vera  toward  the 

cervix  uteri;  pp.,  papillae;  rf,  marginal  furrow.      (Kollmann.) 


comes  completely  embedded  in  the  mucosa,  but,  as  has  been 
pointed  out,  the  chorionic  villi  reach  their  full  development  only 
over  that  portion  of  the  chorion  to  which  the  belly-stalk  is  at- 
tached. The  decidua  which  is  in  relation  to  this  chorion  frondo- 
sum  undergoes  much  more  extensive  modifications  than  that  in 
relation  to  the  chorion  laeve,  and  to  it  the  name  of  decidua  basalts 
{decidua  serotina)  (Fig.  77,  -B)  is  applied,  while  the  rest  of  the  de- 


THE   DECIDUA   VERA 


133 


cidua  which  encloses  the  ovum  is  termed  the  decidua  capsularis 
(decidua  reflexa)  (C) . 

The  changes  which  give  rise  to  the  decidua  vera  may  first  be 
described  and  those  occurring  in  the  others  considered  in  succession. 

(a)  Decidua  vera. — On  opening  a  uterus  during  the  fourth  or 
fifth  month  of  pregnancy,  when  the  decidua  vera  is  at  the  height 
of  its  development,  the  surface  of  the  mucosa  » 

presents  a  corrugated  appearance  and  is 
traversed  by  irregular  and  rather  deep 
grooves  (Fig.  78).  This  appearance  ceases 
at  the  internal  orifice,  the  mucous  membrane 
of  the  cervix  uteri  not  forming  a  decidua, 
and  the  deciduae  of  the  two  surfaces  of  the 
uterus  are  separated  by  a  distinct  furrow 
known  as  the  marginal  groove. 

In  sections  the  mucosa  is  found  to  have 
become  greatly  thickened,  frequently  meas- 
uring I  cm.  in  thickness,  and  its  glands 
have  undergone  very  considerable  modifica- 
tion. Normally  almost  straight  (Fig.  79,  A)y 
they  increase  in  length,   not  only  keeping 


Fig.    79. — ^DiAGRAMMATic  Sections  of  the  Uterine  Mucosa,  A,  in  the  Non- 
pregnant Uterus,  and  B,  at  the  Beginning  of  Pregnancy. 

c,  Stratum  compactum;  gl,  the  deepest  portions  of  the  glands;   m,  muscular  layer; 
sp,  stratum  spongiosum. — {Kundrat  and  Englemann.) 


pace  with  the  thickening  of  the  mucosa,  but  surpassing  its  growth, 
so  that  they  become  very  much  contorted  and  are,  in  addition, 
considerably  dilated  (Fig.  79,  B).  Near  their  mouths  they  are 
dilated,  but  not  very  much  contorted,  while  lower  down  the  reverse 


134  THE   DECIDUA   CAPSULARIS 

is  the  case,  and  it  is  possible  to  recognize  three  layers  in  the^de- 
cidua,  (i)  a  stratum  compactum  nearest  the  lumen  of  the  uterus, 
containing  the  straight  but  dilated  portions  of  the  glands;  (2) 
a  stratum  spongiosum,  so  called  from  the  appearance  which  it 
presents  in  sections  owing  to  the  dilated  and  contorted  portions 
of  the  glands  being  cut  in  various  planes;  and  (3)  next  the  mus- 
ciflar  coat  of  the  uterus  a  layer  containing  the  contorted  but  not 
dilated  extremities  of  the  glands  is  found.  Only  in  the  last  layer 
does  the  epithelium  of  the  glands  retain  its  normal  columnar  form ; 
elsewhere  the  cells,  separated  from  the  walls  of  the  glands,  become 
enlarged  and  irregular  in  shape  and  eventually  degenerate. 

In  addition  to  these  changes,  the  epithelium  of  the  mucosa 
disappears  completely  during  the  first  month  of  pregnancy,  and 
the  tissue  between  the  glands  in  the  stratum  compactum  becomes 
packed  with  large,  often  multinucleated  cells,  which  are  termed  the 
decidual  cells  and  are  probably  derived  from  the  connective  tissue 
cells  of  the  mucosa. 

After  the  end  of  the  fifth  month  the  increasing  size  of  the 
embryo  and  its  membranes  exerts  a  certain  amount  of  pressure  on 
the  decidua,  and  it  begins  to  diminish  in  thickness.  The  portions 
of  the  glands  which  lie  in  the  stratum  compactum  become  more 
and  more  compressed  and  finally  disappear,  while  in  the  spongi- 
osum the  spaces  become  much  flattened  and  the  vascularity  of  the 
whole  decidua,  at  first  so  pronounced,  diminishes  greatly. 

{h)  Decidua  capsularis. — The  decidua  capsularis  has  also  been 
termed  the  decidua  reflexa,  on  the  supposition  that  it  was  formed 
as  a  fold  of  the  uterine  mucosa  reflected  over  the  ovum  after  this 
had  attached  itself  to  the  uterine  wall.  Since,  however,  the 
attachment  of  the  ovum  is  to  be  regarded  as  a  process  of  burrowing 
into  the  uterine  tissues  (see  p.  122),  the  necessity  for  an  upgrowth 
of  a  fold  is  limited  to  an  elevation  of  the  uterine  tissues  in  the 
neighborhood  of  the  ovum  to  keep  pace  with  its  increasing  size. 
Since  it  is  part  of  the  area  of  contact  with  the  ovum  it  possesses 
no  epithelium  upon  the  surface  turned  toward  the  ovum,  although 
in  the  earlier  stages  its  outer  surface  is  covered  by  an  epithehum 
continuous  with  that  of  the  decidua  vera ,  and  between  it  and  the 


THE   DECIDUA  BASALIS  I35 

chorion  there  is  a  portion  of  the  blood  extravasation  in  which  the 
villi  formed  from  the  chorion  laeve  float.  Glands  and  blood-vessels 
also  occur  in  its  walls  in  the  earlier  stages  of  development. 

As  the  ovum  continues  to  increase  in  size  the  capsularis  begins 
to  show  signs  of  degeneration,  these  appearing  first  over  the  pole 
of  the  ovum  opposite  the  point  of  fixation.  Here,  even  in  the 
case  of  the  ovum  described  by  Rossi  Doria,  the  cavity  of  which 
measured  6X5  mm.  in  diameter,  it  has  become  reduced  to  a  thin 
membrane  destitute  of  either  blood-vessels  or  glands,  and  the 
degeneration  gradually  extends  throughout  the  entire  capsule, 
the  portion  of  the  blood  space  which  it  encloses  also  disappearing. 
At  about  the  fifth  month  the  growth  of  the  ovum  has  brought  the 
capsularis  in  contact  throughout  its  whole  extent  with  the  vera, 
and  it  then  appears  as  a  whitish  transparent  membrane  with  no 
trace  of  either  glands  or  blood-vessels,  and  it  eventually  disappears 
by  fusing  with  the  vera. 

(c)  Decidua  hasalis. — The  structure  of  the  decidua  basalis,  also 
known  as  the  decidua  serotina,  is  practically  the  same  as  that  of 
the  vera  up  to  about  the  fifth  month.  It  differs  only  in  that,  being 
part  of  the  area  of  contact  of  the  ovum,  it  loses  its  epithelium 
much  earlier  and  is  also  the  seat  of  extensive  blood  extravasations, 
due  to  the  erosion. of  its  vessels  by  the  chorionic  trophoblast.  Its 
glands,  however,  undergo  the  same  changes  as  those  of  the  vera, 
so  that  in  it  also  a  compactum  and  a  spongiosum  may  be  recog- 
nized. Beyond  the  fifth  month,  however,  there  is  a  great  differ- 
ence between  it  and  the  vera,  in  that,  being  concerned  with  the 
nutrition  of  the  embryo,  it  does  not  partake  of  the  degeneration 
noticeable  in  the  other  deciduae,  but  persists  until  birth,  forming  a 
part  of  the  structure  termed  the  placenta. 

The  Placenta. — This  organ,  which  forms  the  connection  be- 
tween the  embryo  and  the  maternal  tissues,  is  composed  of  two 
parts,  separated  by  the  intervillous  space.  One  of  these  parts 
is  of  embryonic  origin,  being  the  chorion  frondosum,  while  the 
other  belongs  to  the  maternal  tissues  and  is  the  decidua  basalis. 
Hence  the  terms  placenta  fetalis  and  placenta  uterina  frequently 
applied  to  the  two  parts.     The  fully  formed  placenta  is  a  more 


136  '        THE   PLACENIA 

or  less  discoidal  structure,  convex  on  the  surface  next  the  uterine 
muscularis  and  concave  on  that  turned  toward  the  embryo,  the 
umbilical  cord  being  continuous  with  it  near  the  center  of  the 
latter  surface.  It  averages  about  3.5  cm.  in  thickness,  thinning 
out  somewhat  toward  the  edges,  and  has  a  diameter  of  15  to  20 
cm.,  and  a  weight  varying  between  500  and  1250  grams.  It  is 
situated  on  one  of  the  surfaces  of  the  uterus,  the  posterior  more 
frequently  than  the  anterior,  and  usually  much  nearer  the  fundus 
than  the  internal  orifice.  It  develops,  in  fact,  wherever  the  ovum 
happens  to  become  attached  to  the  uterine  walls,  and  occasionally 
this  attachment  is  not  accomplished  until  the  ovum  has  descended 
nearly  to  the  internal  orifice,  in  which  case  the  placenta  may  com- 
pletely close  this  opening  and  form  what  is  termed  a  placenta 
prcBvia. 

If  a  section  of  a  placenta  in  a  somewhat  advanced  stage  of  de- 
velopment be  made,  the  following  structures  may  be  distinguished : 
On  the  inner  surface  there  will  be  a  delicate  layer  representing  the 
amnion  (Fig.  80,  Am),  and  next  to  this  a  somewhat  thicker  one 
which  is  the  chorion  (Cho) ,  in  which  the  degenerative  changes  al- 
ready mentioned  may  be  observed.  Succeeding  this  comes  a 
much  broader  area  composed  of  the  large  intervillous  blood  space 
in  which  lie  sections  of  the  villi  (vi)  cut  in  various  directions. 
Then  follows  the  stratum  compactum  of  the  basalis,  next  the 
stratum  spongiosum  (D^),  next  the  outermost  layer  of  the  mucosa 
(Z>")>  ill  which  the  uterine  glands  retain  their  epithelium,  and, 
finally  the  muscularis  uteri  (Mc). 

These  various  structures  have,  for  the  most  part,  been  already 
described  and  it  remains  here  only  to  say  a  few  words  concerning 
the  special  structure  of  the  basal  compactum  and  concerning 
certain  changes  that  take  place  in  the  intervillous  space. 

The  stratum  compactum  of  the  basal  decidua  forms  what  is 
termed  the  basal  plate  of  the  placenta,  closing  the  intervillous  space 
on  the  uterine  side  and  being  traversed  by  the  maternal  blood- 
vessels that  open  into  the  space.  The  formation  of  canalized 
fibrin,  already  mentio^ied  in  connection  with  the  decidua  vera  and 
the  syncytium  of  the  villi,  also  occurs  in  the  basal  portion  of  the 


THE  PLACENTA 


137 


Fig.  80. — Section   through   a   Placenta   of   Seven   Months'    Development. 

Am,  Amnion;  cho,  chorion;  D,  layer  of  decidua  containing  the  uterine  glands; 
Mc,  muscular  coat  of  the  uterus;  Ve,  maternal  blood-vessel;  Vi,  stalk  of  a  villus;  vi, 
villi  in  section.^ — (Minoi.) 


138 


THE  PLACENTA 


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SEPARATION    OF   THE   DECIDU^  I39 

decidua,  a  definite  layer  of  it,  known  as  Nitabuch^s  fibrin  stria, 
being  a  characteristic  constituent  of  the  basal  plate  and  patches 
of  greater  or  less  extent  also  occur  upon  the  surface  of  the  plate. 
Leucocytes  also  occur  in  considerable  abundance  in  the  plate 
and  their  presence  has  been  taken  to  indicate  an  attempt  on  the 
part  of  the  maternal  tissues  to  resist  the  erosive  action  of  the 
parasitic  ovum.  From  the  surface  of  the  basal  plate  processes, 
termed  placental  septa,  project  into  the  intervillous  space,  group- 
ing the  villi  into  cotyledons  and  giving  attachment  to  some 
of  the  fixation  villi  (Fig.  81).  Throughout  the  greater  extent 
of  the  placenta  the  septa  do  not  reach  the  surface  of  the  chorion, 
but  at  the  periphery,  throughout  a  narrow  zone,  they  do  come 
into  contact  with  the  chorion  and  unite  beneath  it  to  form  a 
membrane  which  has  been  termed  the  closing  plate.  Beneath 
this  lies  the  peripheral  portion  of  the  intervillous  space,  which, 
ow'ng  to  the  arrangement  of  the  septa  in  this  region,  appears  to 
be  imperfectly  separated  from  the  rest  of  the  space  and  forms 
what  is  termed  the  marginal  sinus  (Fig.  81). 

Attention  has  already  been  called  to  the  formation  of  canalized 
fibrin  or  fibrinoid  in  connection  with  the  syncytium  of  the  villi. 
In  the  later  stages  of  pregnancy  there  may  be  produced  by  this 
process  masses  of  fibrinoid  of  considerable  size,  lying  in  the  inter- 
villous space;  these,  on  account  of  their  color,  are  termed  white 
infarcts  and  may  frequently  be  observed  as  whitish  or  grayish 
patches  through  the  walls  of  the  placenta  after  its  expulsion. 
Red  infarcts  produceid  by  the  clotting  of  the  blood,  also  occur, 
but  with  much  less  regularity  and  frequency. 

The  Separation  of  the  Deciduae  at  Birth.— At  parturition, 
after  the  rupture  of  the  amnion  and  the  expulsion  of  the  fetus, 
there  still  remain  in  the  uterine  cavity  the  deciduse  and  the  amnion, 
which  is  in  contact  but  not  fused  with  the  deciduae.  A  continu- 
ance of  the  uterine  contractions,  producing  what  are  termed  the 
"after-pains,"  results  in  the  separation  of  the  placenta  from  the 
uterine  walls,  the  separation  taking  place  in  the  deep  layers  of  the 
spongiosum,  so  that  the  portion  of  the  mucosum  which  contains 
the    undegenerated    glands    remains   behind.     As    soon   as    the 


I40  SEPARATION    OF   THE   DECIDU^ 

placenta  has  separated,  the  separation  of  the  decidua  vera  takes 
place  gradually  though  rapidly,  the  Hne  of  separation  again  being 
in  the  deeper  layers  of  the  stratum  spongiosum,  and  the  whole 
of  the  deciduae,  together  with  the  amnion,  is  expelled  from  the 
uterus  forming  what  is  known  as  the  '' after-birth." 

•  Hemorrhage  from  the  uterine  vessels  during  and  after  the 
separation  of  the  deciduae  is  prevented  by  the  contractions  of  the 
uterine  walls,  assisted,  according  to  some  authors,  by  a  pre- 
liminary blocking  of  the  mouths  of  the  uterine  vessels  by  certain 
large  polynuclear  decidual  cells  found  during  the  later  months  of 
pregnancy  in  the  outer  layers  of  the  decidua  basalis.  The  re- 
generation of  the  uterine  mucosa  after  parturition  has  its  start- 
ing-point from  the  epithehum  of  the  undegenerated  glands  which 
persist,  this  epithelium  rapidly  evolving  a  complete  mucosa  over 
the  entire  surface  of  the  uterus. 

The  complicated  arrangement  of  the  human  placenta  is,  of  course, 
the  culmination  of  a  long  series  of  specializations,  the  path  along  which 
these  have  proceeded  being  probably  indicated  by  the  conditions  ob- 
taining in  some  of  the  lower  mammals.  The  Monotremes  resemble 
the  reptiles  in  being  oviparous  and  in  this  group  of  forms  there  is  no 
relation  of  the  ovum  to  the  maternal  tissues  such  as  occurs  in  the 
formation  of  a  placenta.  In  the  other  mammals  viviparity  is  the 
rule  and  this  condition  does  demand  some  sort  of  connection  between 
the  fetal  and  maternal  tissues.  One  of  the  simplest  of  such  con- 
nections is  that  seen  in  the  pig,  where  the  chorionic  villi  of  the  ovum 
fit  into  corresponding  depressions  in  the  uterine  mucosa,  this  tissue, 
however,  undergoing  no  destruction,  and  at  birth  the  villi  simply 
withdraw  from  the  depressions  of  the  mucosa,  leaving  it  intact.  This 
type  of  placentation  is  an  embryotrophic  one,  and  since  there  is  no 
separation  of  deciduae  from  the  uterine  wall  after  pregnancy  it  is  also 
of  the  indeciduate  type.  In  the  sheep  the  placentation  is  also  embryo- 
trophic  and  indeciduate,  but  destruction  of  the  maternal  mucosa  does 
take  place,  the  villi  penetrating  deeply  into  it  and  coming  into  rela- 
tion with  the  connective  tissue  surrounding  the  maternal  blood-vessels. 
Another  step  in  advance  is  shown  by  the  dog,  in  which  even  the 
connective  tissue  around  the  maternal  vessels  in  the  placental  area 
undergoes  almost  complete  destruction  so  that  the  chorionic  villi  are 
separated  from  the  maternal  blood  practically  only  by  the  endothelial 
lining  of  the  maternal  vessels.  In  this  case  the  mucosa  undergoes  so 
much  alteration  that  the  undestroyed  portions  of  it  are  sloughed  off 
after  birth  as  a  decidua,  so  that  the  placentation,  like  that  in  man,  is 


t 


LITERATURE  I41 

of  the  deciduate  type.  It  still  represents,  however,  an  embryotrophic 
type,  although  closely  approximating  to  the  haemotrophic  one  found 
in  man,  in  which,  as  described  above,  the  destruction  of  the  maternal 
tissues  proceeds  so  far  as  to  open  into  the  maternal  blood-vessels,  so 
that  the  fetal  villi  are  in  direct  contact  with  the  maternal  blood. 

If  these  various  stages  may  be  taken  to  represent  steps  by  which 
the  conditions  obtaining  in  the  human  placenta  have  been  evolved,  the 
entire  process  may  be  regarded  as  the  result  of  a  progressive  activity 
of  a  parasitic  ovum.  In  the  simplest  stage  the  pabulum  supplied  by 
the  uterus  was  sufficient  for  the  nutrition  of  the  parasite,  but  gradu- 
ally the  ovum,  by  means  of  its  plasmodi-trophoblast,  began  to  attack 
the  tissues  of  its  host,  thus  obtaining  increased  nutrition,  until  finally, 
breaking  through  into  the  maternal  blood-vessels,  it  achieved  for  itself 
still  more  favorable  nutrition,  by  coming  into  direct  contact  with  the 
maternal  blood. 

LITERATURE 

In  addition  to  the  papers  by  Beneke  and  Strahl,  Bryce  and  Teacher,  Frassi, 
Jung,  Herzog,  Grosser  and  Linzenmeier  cited  in  Chapter  III,  the  following  may  be 
mentioned: 
A.  Branca:  "Recherches  sur  la  structure,  revolution  et  le  r61e  de  la  vesicule  om- 

bilicale  de  I'homme"  Joiirn.  de  VAnat.  el  de  la  Physiol.,  xldc,  1913. 
E.Cova:  "Ueber  ein  menschliches  Ei  der  zweiten  Woohe:,'' Arch,  fur  Gynaek.,  lxxxiii, 

1907. 
A.  Debeyre:  "Description  d'un  embryon  humain  de  0.9  mm.,"  Journ.  de  VAnat.  et 

de  la  Physiol,  xlviii,  i 9 i  2 . 
L.  Frassi:  "Ueber  ein  junges  menschliches  Ei  in  situ,"  Arch.  fUr  mikr.  Anat.,  lxx, 

1907. 
O.  Grosser:  " Vergleichende  Anatomie  und  Entwicklungsgeschichte  der  Eihaute 

und  der  Placenta  mit  besonderer  Beriicksichtigung  des  Menschen,"  Wien,  1909. 
H.  Happe:  " Beobachtungen  an  Eihauten  junger  menschlicher  Eier,"  Anat.  Hefte, 

xxxii,  1906. 
W.  His:  "Die  Umschliessung  der  menschlichen  Frucht  wahrend  der  fruhesten  Zeit 

des  Schwangerschafts,"  Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1897. 
M.  Hofmeier:  "Die  menschliche  Placenta,"  Wiesbaden,  1890. 
R.  W.  Johnstone:  "Contribution  to  the  study  of  the  early  human  ovum,"  Journ. 

Ohstet.  and  Gynaek.,  xxvi,  1914. 
F.  Keibel:  "Zur  Entwickelungsgeschichte  der  Placenta,"  Anat.  Anzeiger,  iv,  1889. 

F,  Keibel:  "Ueber  die  Grenze  zwischen  miitterlichen  und  fetalen  Gewebe,"  Anat. 

Anzeiger,  xlviii,  191 5. 
J.  Kollmann:  "Die  menschlichen  Eier  von  6  mm.  Grosse,"  Archiv  fur  Anat.  und 

Physiol.,  Anat.  Abth.,  1879. 
T.  G.  Lee:  "Implantation  of  the  ovum  in  Spermophilus  tridecemlineatus  Mitch." 

MarklAnniversary  Volume,  New  York,  1904. 

G.  Leopold:  "Ueber  ein  sehr  junges  menschliches  Ei  in  situ,"  Arb.  aus  der  konigl. 

Frauenklinik  in  Dresden,  w,  1906. 


142       \.  LITERATURE. 

F.  Marchand:  "^Btobaehtungen  an  jungen  menschlichen  Eiern,"  Anat.  Hefte,  xxi, 

1903. 

J.  Merttens:  "Beitrage  zur  normalen  und  pathologischen  Anatomic  der  mensch- 
lichen Placenta,"  Zeitschriftfiir  Geburtshulfe  und  Gynaekol.,  xxx  and  xxxi,  1894. 

C.  S.  Minot:  "Uterus  and  Embryo,"  Journal  of  Morphol.,  11,  1889. 

G.  Paladino:  "Sur  la  genese  des  espaces  intervilleux  du  placenta  humain  et  de  leur 

premier  contenu,  comparativement  d  la  meme  partie  chez  quelques  mammiferes, 

Archives  Ital.  de  Biolog.,  xxxi  and  xxxii,  1899. 
H.  Peters:  "Ueber  die  Einbettung  des  menschlichen  Eies  und  das  friiheste  bisher 

bekannte  menschliche  Placentationsstadium,"  Leipzig  und  Wien,  1899. 
J.  Rejsek:  "Anheftung  (Implantation)  des  Saiigetiereies  an  die  Uteruswand,  insbe- 

sondere  des  Eies  von  Spermophilus  citellus/Mrc/f./wr  mikrosk.  Anat.,  lxiii,  1904. 
T.  Rossi  Doria:  "Ueber  die  Einbettung  des  menschlichen  Eies,  studirt  an  einem 

kleinen  Eie  der  zweiten  Woche,"  Arch,  fiir  Gynaek.,  lxxvi,  1905. 
C.  Ruge:  "Ueber  die  menschliche  Placentation,"  Zeitschrijt  fiir  Geburtshulfe  und 

Gynaekol,  xxxix,  1898. 
Siegenbeek  VAN  Heukelom:  "Ueber  die  menschliche  Placentation,"  Arch.  f.  Anat. 

und  Phys.,  Anat.  Abth.,  1898. 
F.  Graf  Spee:  "Ueber  die  menschliche  Eikammer  und  Decidua  reflexa,"  Verhandl. 

des  Anat.  Gesellsch.,  xii,  1898. 
H.  Strahl:  "Die  menschliche  Placenta,"  Ergebn.  der  Anat.  und  Entwickl.,  ii,  1893. 

"Neues  iiber  den  Bau  der  Placenta,"  ibid,  vi,  1897. 

"Placentaranatomie,"  ibid. ,vin,  1899. 
R.  ToDYO:  "Ein  junges  menschliches  Ei,"  Arch.  fUr  Gynaek.,  xcv,  1912. 
Van  Cauwenberghe:  "Recherches  sur  la  role  du  Syncytium  dans  la    nutrition 

embryonnaire  de  la  femme,"  Arch,  de  Biol.,  xxiii,  1907. 
J.  C.  Webster:  "Human  Placentation,"  Chicago,  1901. 
E.  Wormser:  "Die  Regeneration  der  Uterusschleimhaut  nach  der  Geburt,"  Arch. 

fiir  Gynaek.,  Lxrx,  1903. 


PART  II 

ORGANOGENY 


CHAPTER  VI 

THE  DEVELOPMENT  OF  THE  INTEGUMENTARY 

SYSTEM 

The  Development  of  the  Skin. — The  skin  is  composed  of  two 
embryologically  distinct  portions,  the  outer  epidermal  layer  being 
developed  from  the  ectoderm,  while  the  dermal  layer  is  mesen- 
chymatous  in  its  origin. 

The  ectoderm  covering  the  general  surface  of  the  body  is,  in  the 
earliest  stages  of  development,  a  single  layer  of  cells,  but  at  the 
end  of  the  first  month  it  is  composed  of  two  layers,  an  outer  one, 
the  epitrichium,  consisting  of  slightly  flattened  cells,  and  a  lower 
one  whose  cells  are  larger,  and  which  will  give  rise  to  the  epidermis 
(Fig.  82,  A).  During  the  second  month  the  differences  between 
the  two  layers  become  more  pronounced,  the  epitrichial  cells 
assuming  a  characteristic  domed  form  and  becoming  vesicular  in 
structure  (Fig.  82,  B).  These  cells  persist  until  about  the  sixth 
month  of  development,  but  after  that  they  are  cast  off,  and, 
becoming  mixed  with  the  secretion  of  sebaceous  glands  which 
have  appeared  by  this  time,  form  a  constituent  of  the  vernix 
caseosa. 

In  the  meantime  changes  have  been  taking  place  in  the  epi- 
dermal layer  which  result  in  its  becoming  several  layers  thick 
(Fig.  82,  B),  the  innermost  layer  being  composed  of  cells  rich  in 
protoplasm,  while  those  of  the  outer  layers  are  irregular  in  shape 
and  have  clearer  contents.  As  development  proceeds  the  number 
of  layers  increases  and  the  superficial  ones,  undergoing  a  horny 
degeneration,  give  rise  to  the  stratum  corneum,  while  the  deeper 

143 


144 


DEVELOPMENT    OF   THE    SKIN 


ones  become  the  stratum  Malpighii.  At  about  the  fourth  month 
ridges  develop  on  the  under  surface  of  the  epidermis,  projecting 
downward  into  the  dermis,  and  later  secondary  ridges  appear  in 
the  intervals  between  the  primary  ones,  while  on  the  palms  and 
soles  ridges  appear  upon  the  outer  surface  of  the  epidermis,  corre- 
sponding in  position  to  the  primary  ridges  of  the  under  surface. 

The  mesenchyme  which  gives  rise  to  the  dermis  grows  in  from 
all  sides  between  the  epidermis  and  the  outer  layer  of  the  myo- 
tomes, which  are  at  first  in  contact,  and  forms  a  continuous  layer 


Fig.  82. — A,   Section  of  Skin  from  the  Dorsum  of  Finger  of  an  Embryo  of 

4.5  CM,  B,  from  the  Plantar  Surface  of  the  Foot  of  an  Embryo  of  10.2  cm. 

et,  Epitrichium;  ep,  epidermis. 

underlying  the  epidermis  and  showing  no  indications  of  a  seg- 
mental arrangement.  It  becomes  converted  principally  into 
fibrous  connective  tissue,  the  outer  layers  of  which  are  relatively 
compact,  while  the  deeper  ones  are  looser,  forming  the  subcu- 
taneous areolar  tissue.  Some  of  the  mesenchymal  cells,  how- 
ever, become  converted  into  non-striated  muscle-fibers,  which  for 
the  most  part  are  few  in  number  and  associated  with  the  hair 
follicles,  though  in  certain  regions,  such  as  the  skin  of  the  scrotum, 
they  are  very  numerous  and  form  a  distinct  layer  known  as  the 
dartos.     Some  cells  also  arrange  themselves  in  groups  and  undergo 


DEVELOPMENT    OF   THE    NAILS 


145 


a  fatty  degeneration,  well-defined  masses  of 
adipose  tissue  embedded  in  the  lower  layers 
of  the  dermis  being  thus  formed  at  about 
the  sixth  month. 

Although  the  dermal  mesenchyme  is  unseg- 
mental  in  character,  yet  the  nerves  which  send 
branches  to  it  are  ■  segmental,  and  it  might  be 
expected  that  indications  of  this  condition  would 
be  retained  by  the  cutaneous  nerves  even  in  the 
adult.  A  study  of  the  cutaneous  nerve-supply  in 
the  adult  realizes  to  a  very  considerable  extent 
this  expectation,  the  areas  supplied  by  the  vari- 
ous nerves  forming  more  or  less  distinct  zones, 
and  being  therefore  segmental  (Fig.  83).  But  a 
considerable  commingling  of  adjacent  areas  has 
also  occurred.  Thus,  while  the  distribution  of 
the  cutaneous  branches  of  the  fourth  thoracic 
nerve, as  determined  experimentally  in  the  monkey 
(Macacus),  is  distinctly  zonal  or  segmental,  the 
nipple  lying  practically  in  the  middle  line  of  the 
zone,  the  upper  half  of  its  area  is  also  supplied  or 
overlapped  by  fibers  of  the  third  nerve  and  the 
lower  half  by  fibers  of  the  fifth  (Fig.  84),  so  that 
any  area  of  skin  in  the  zone  is  innervated  by 
fibers  coming  from  at  least  two  segmental  nerves 
(Sherrington).  And,  furthermore,  the  distribu- 
tion of  each  nerve  crosses  the  mid-ventral  line 
of  the  body,  forming  a  more  or  less  extensive 
crossed  overlap. 

And  not  only  is  there  a  confusion  of  adjacent 
areas  but  an  area  may  shift  its  position  relatively 
to  the  deeper  structures  supplied  by  the  same 
nerve,  so  that  the  skin  over  a  certain  muscle  is 
not  necessarily  supplied  by  fibers  from  the  nerve 
which  supplied  the  muscle.  Thus,  in  the  lower 
half  of  the  abdomen,  the  skin  at  any  point  will 
be  supplied  by  fibers  from  higher  nerves  than  those 
supplying  the  underlying  muscles  (Sherrington), 
and  the  skin  of  the  limbs  may  receive  twigs  from 
nerves  which  are  not  represented  at  all  in  the 
muscle-supply  (second  and  third  thoracic  and 
third  sacral). 

The  Development   of   the  Nails.  — The 

earliest  indications  of  the  development  of ^ the 
10 


7> 


rj\ 


rs 


Te 


T9 


Fro 


^Tn 


Sz 


Lt 


U 


Sf 

Fig.  83. — Diagram 
showing  the  cuta- 
NEOUS Distribution 
OF  THE  Spinal  Nerves. 
—{Head.) 


146 


DEVELOPMENT    OF   THE    NATLS 


nails  have  been  described  by  Zander  in  embryos  of  about 
nine  weeks  as  slight  thickenings  of  the  epidermis  of  the  tips 
of  the  digits,  these  thickenings  being  separated  from  the  neigh- 
boring tissue  by  a  faint  groove.    Later  the  nail  areas  migrate 


i 


VWhi 


\\\m\\mm\\mw 


Fig.  84. — Diagram  showing  the  Overlap  of  the  ///,  IV,  and  V  Intercostal 
Nerves  of  a  Monkey. — {Sherrington.) 


.nf 


^ttcl^l^^^^fe-^O-^^&.^r-^^^ 


Pig.  85. — Longitudinal  Section  through  the  Terminal  Joint  of  the  Index- 
finger  OF  AN  Embryo  of  4.5  cm. 
e.  Epidermis;  ep,  epitrichium;  «/,  nail  fold;  Ph,  terminal  phalanx;  sp,    sole  plate. 

to  the  dorsal  surfaces  of  the  terminal  phalanges  (Fig.  85)  and 
the  grooves  surrounding  the  areas  deepen,  especially  at  their 
proximaLedges,  where  they  form  the  nail-folds  (nf),  while  distally 
thickenings  of  the  epidermis  occur  to  form  what  have  been  termed 


DEVELOPMENT   OF   THE    NAILS 


147 


sp- 


sc 


ep 


sole-plates  {sp),  structures  quite  rudimentary  in  man,  but  largely 
developed  in  the  lower  animals,  in  which  they 
form  a  considerable  portion  of  the  claws. 

The  actual  nail  substance  does  not  form, 
however,  until  the  embryo  has  reached  a  length 
of  about  17  cm.  By  this  time  the  epidermis 
has  become  several  layers  thick  and  its  outer  m^Jlb 

layers,  over  the  nail  areas  as  well  s  elsewhere, 
have  become  transformed  into  ^he  stratum 
corneum  (Fig.  86,  sc),  and  it  is  in  the  deep  layers 
of  this  (the  stratum  lucidum)  that  keratin 
granules  develop  in  cells  which  degenerate  to 
give  rise  to  the  nail  substance  {n) .  At  its  first 
formation,  accordingly  the  nail  is  covered  by  the 
outer  layers  of  the  stratum  corneum  as  well  as 
by  the  epitrichium,  the  two  together  forming 
what  has  been  termed  the  eponychium  (Fig.; 
86,  ep).  The  epitrichium  soon  disappears,  how-^ 
ever,  leaving  only  the  outer  layers  of  the  stratum 
corneum  as  a  covering,  and  this  also  later  dis- 
appears with  the  exception  of  a  narrow  band 
surrounding  the  base  of  the  nail  which  persists 
as  the  perionyx. 

The  formation  of  the  nail  begins  in  the  more 
proximal  portion  of  the  nail  area  and  its  further 
growth  is  by  the  addition  of  new  keratinized 
cells  to  its  proximal  edge  and  lower  surface, 
these  cells  being  formed  only  in  the  proximal 
part  of  the  nail  bed  in  a  region  marked  by  its 
whitish  color  and  termed  the  lunula. 


The  first  appearance  of  the  nail  areas  at  the  tips 
of  the  digits  as  described  by  Zander  has  not  yet 
been  confirmed  by  later  observers,  but  the  migra- 
tion of  the  areas  to  the  dorsal  surfaces  necessitated  by 
such  a  location  of  the  primary  differentiation  affords 
an  explanation  of  the  otherwise  anomalous  cutaneous  nerve-supply 
of  the  nail  areas  in  the  adult,  this  being  from  the  palmar  (plantar)  nerves. 


Fig.  86. — ^Longi- 
tudinal      Section 

THROUGH   THE    NaIL 

Area  in  an  Embryo 

OF  17  CM. 

ep,  Eponychium; 
n,  nail  substance;  nb, 
nail  bed;  sc,  stratum 
corneum;  sp,  sole 
plate. — (jOkamura.) 


148 


DEVELOPMENT    OF   THE   HAIRS 


The  Development  of  the  Hairs. — The  hairs  begin  to  develop 
at  about  the  third  month  and  continue  to  be  formed  during  the 
remaining  portions  of  fetal  life.  They  arise  as  solid  cylindrical 
downgrowths,  projecting  obliquely  into  the  subjacent  dermis  from 
the  lower  surface  of  the  epidermis.  As  these  downgrowths  con- 
tinue to  elongate,  they  assume  a  somewhat  club-shaped  form 
(Fig.  87,  A),  and  later  the  extremity  of  each  club  moulds  itself 


m. 


L  hj 


Fig.  87. — The  Development  of  a  Hair. 
c.  Cylindrical  cells  of  stratum  mucosum;  hj,  wall  of  hair  follicle;  w,  mesoderm; 
mu,  stratum  mucosum  of  epidermis;  p,  hair  papilla;  r,  root  of  hair;  5,  sebaceous 
gland. — (Kollmann.) 

over  the  summit  of  a  small  papilla  which  develops  from  the 
dermis  (Fig.  87,  -S).  Even  before  the  dermal  papilla  has  made  its 
appearance,  however,  a  differentiation  of  the  cells  of  the  down- 
growth  becomes  evident,  the  central  cells  becoming  at  first  spindle- 
shaped  and  then  undergoing  a  keratinization  to  form  the  hair 
shaft,  while  the  more  peripheral  ones  assume  a  cuboidal  form  and 


DEVELOPMENT    OF   THE    HAIRS  1 49 

constitute  the  lining  of  the  hair  follicle.  The  further  growth  of 
the  hair  takes  place  by  the  addition  to  its  basal  portion  of  new 
keratinized  cells,  probably  produced  by  the  multiplication  of  the 
epidermal  cells  which  envelop  the  papilla. 

From  the  cells  which  form  the  lining  of  each  follicle  an  out- 
growth takes  place  into  the  surrounding  dermis  to  form  a  se- 
baceous gland,  which  is  at  first  solid  and  club-shaped,  though 
later  it  becomes  lobed.  The  central  cells  of  the  outgrowth 
separate  from  the  peripheral  and  from  one  another,  and,  their 
protoplasm  undergoing  a  fatty  degeneration,  they  finally  pass  out 
into  the  space  between  the  follicle  walls  and  the  hair  and  so  reach 
the  surface,  the  peripheral  cells  later  giving  rise  by  division  to 
new  generations  of  central  cells.  During  fetal  life  the  fatty 
material  thus  poured  out  upon  the  surface  of  the  body  becomes 
mingled  with  the  cast-off  epitrichial  cells  and  constitutes  the 
white  oleaginous  substance,  the  vernix  caseosa,  which  covers  the 
surface  of  the  new-born  child.  The  muscles,  arrectores  pilorum, 
connected  with  the  hair  follicles  arise  from  the  mesenchyme  cells 
of  the  surrounding  dermis. 

The  first  growth  of  hair  forms  a  dense  covering  over  the  entire 
surface  of  the  fetus,  the  hairs  which  compose  it  being  exceedingly 
fine  and  silky  and  constituting  what  is  termed  the  lanugo.  This 
growth  is  cast  off  soon  after  birth,  except  over  the  face,  where  it 
is  hardly  noticeable  on  account  of  its  extreme  fineness  and  lack 
of  coloration.  The  coarser  hairs  which  replace  it  in  certain  regions 
of  the  body  probably  arise  from  new  follicles,  since  the  formation 
of  follicles  takes  place  throughout  the  later  periods  of  fetal  life 
and  possibly  after  birth.  But  even  these  later  formed  hairs  do 
not  individually  persist  for  any  great  length  of  time,  but  are  con- 
tinually being  shed,  new  or  secondary  hairs  normally  developing 
in  their  places.  The  shedding  of  a  hair  is  preceded  by  a  cessation 
of  the  proliferation  of  the  cells  covering  the  dermal  papilla  and  by 
a  shrinkage  of  the  papilla,  whereby  it  becomes  detached  from  the 
hair,  and  the  replacing  hair  arises  from  a  papilla  which  is  prob- 
ably budded  off  from  the  older  one  before  its  degeneration  and 
carries  with  it  a  cap  of  epidermal  cells. 


ISO 


DEVELOPMENT  OF  THE  SUDORIPAROUS  GLANDS 


It  is  uncertain  whether  the  cases  of  excessive  development  of  hair 
over  the  face  and  upper  part  of  the  body  which  occasi  onally  occur  are 
due  to  an  excessive  development  of  the  later  hair  follicles  (hyper- 
trichosis) or  to  a  persistence  and  continued  growth  of  the  lanugo. 

The  Development  of  the  Sudoriparous  Glands. — The  sudori- 
parous glands  arise  during  the  fifth  month  as  solid  cylindrical 
outgrowths  from  the  primary  ridges  of  the  epidermis  (Fig.  88), 
and  at  first  project  vertically  downward  into  the  subjacent  dermis. 
Later,  however,  the  lower  end  of  each  downgrowth  is  thrown  into 
coils,  and  at  the  same  time  a  lumen  appears  in  the  center.     Since, 

however,  the  cylinders  are  formed 

from   the   deeper  layers   of  the 

epidermis,  their    lumina  do  not 

at  first  open  upon  the  surface, 

1 M 1 '^VT^'^u L^^^  ^     ^^^  gradually  approach  it  as  the 

't^--S  I'li^^  ^^^^^  ^^  ^^^  deeper  layers  of  the 

'(  U/TJ,  LMiyn^XHM£V  /  V  epidermis    replace    those  which 

are  continually  being  cast  off 
from  the  surface  of  the  stratum 
corneum.  The  final  opening  to 
the  surface  occurs  during  the 
seventh  month  of  development. 

The  Development  of  the 
Mammary  Glands. — In  the  ma- 
jority of  the  lower  mammals  a  number  of  mammary  glands  occur, 
arranged  in  two  longitudinal  rows,  and  it  has  been  observed  that 
in  the  pig  the  first  indication  of  their  development  is  seen  in  a 
thickening  of  the  epidermis  along  a  line  situated  at  the  junction  of 
the  abdominal  walls  with  the  membrana  reuniens  (Schulze). 
This  thickening  subsequently  becomes  a  pronounced  ridge,  the 
milk  ridge,  from  which,  at  certain  points,  the  mammary  glands 
develop,  the  ridge  disappearing  in  the  intervals.  In  human 
embryos  8  mm.  or  less  in  length  a  similar  epidermal  thickening 
has  been  observed  extending  from  just  below  the  axilla  to  the 
inguinal  region  (Fig.  89).  Later,  in  embryos  of  10  to  13  mm.,  the 
anterior  part  of  the  thickening  becomes  more  distinct  while  its 
lower  two-thirds  become  less  distinct  and  eventually  disappear. 


Fig.  88. — ^Lower  Surface  of  a  De- 
tached Portion   of   Epidermis  from 
THE  Dorsum  of  the  Hand. 
h,  Hair   follicle:  s,  sudoriparous  gland, 
—{Blaschko.) 


DEVELOPMENT   OF   THE    MAMMARY    GLANDS 


151 


In  somewhat  older  embryos  (14  to  18  mm.)  the  gland  is  rep- 
resented by  a  marked  thickening  of  the  epidermis  which  projects 
down  into  the  dermis  and  has  a  circular  outline  (Fig.  90,  A). 
Later  the  thickening  becomes  lobed  (Fig.  90,  B),  and  its  superficial 
and  central  cells  become  cornified  and  are  cast  off,  so  that  the 
gland  area  appears  as  a  depression  of  the  surface  of  the  skin. 
During  the  fifth  and  sixth  months  the  lobes  elongate  into  solid 
cylindrical  columns  of  cells  (Fig.  91)  resembling  not  a  little  the 
cylinders  which  become  con- 
verted into  sudoriparous  glands, 
and  each  column  becomes 
slightly  enlarged  at  its  lower 
end,  from  wh  ich  outgrowths  be- 
gin to  develop  to  form  the 
acini.  A  lumen  first  appears 
in  the  lower  ends  of  the  col- 
umns and  is  formed  by  the 
separation  and  breaking  down 
of  the  central  cells,  the  peri- 
pheral cells  persisting  as  the 
lining  of  the  acini  and  ducts. 

The    elevation  of  the  gland    Fig.  89. — Milk  Ridge  (mr)  in  a  Human 
1  ,1  r  ,      r  Embryo. — (Kallius.) 

area  above  the  surface  to  form 

the  nipple  appears  to  occur  at  different  periods  in  different  embryos 
and  frequently  does  not  take  place  until  after  birth.  In  the  re- 
gion around  the  nipple  sudoriparous  and  sebaceous  glands  develop, 
the  latter  also  occurring  within  the  nipple  area  and  fre- 
quently opening  into  the  extremities  of  the  lacteal  ducts.  In  the 
areola,  as  the  area  surrounding  the  nipple  is  termed,  other  glands 
known  as  Montgomery's  glands,  also  appear,  their  development 
resembling  that  of  the  mammary  gland  so  closely  as  to  render  it 
probable  that  they  are  really  rudimentary  mammary  glands, 
perhaps  developments  of  portions  of  the  original  milk  ridge  other 
than  that  which  gives  rise  to  the  main  gland. 

The  further  development  of  the  glands,  consisting  of  an  in- 
crease in  the  length  of  the  ducts  and  the  development  from  them  of 


152  DEVELOPMENT    OF   THE    MAMMARY   GLANDS 

additional  acini,  continues  slowly  up  to  the  time  of  puberty  in  both 
sexes,  but  at  that  period  further  growth  usually  ceases  in  the  male, 
while  in  females  it  continues  for  a  time  and  the  subjacent  dermal 
tissues,  especially  the  adipose  tissue,  undergo  a  rapid  development. 

The  occurrence  of  a  milk  ridge  in  human  embryos  is  of  special  inter- 
est in   connection  with   the  occasional  formation  of  supernumerary 


'^^^^--^■'  -■ ■.:.     ^ 

Fig.  90. — Sections  through  the  Epidermal  Thickenings  which  Represent 
THE  Mammary  Gland  in  Embryos  (A)  of  6  cm.  and  (B)  of  10.2    cm. 

mammary  glands  (polymastia).  This  is  by  no  means  an  infrequent 
anomaly;  it  has  been  observed  in  19  per  cent,  of  over  100,000  soldiers 
of  the  German  army  and  in  47  per  cent,  of  the  individuals  of  certain 
regions  of  Germany.  The  anomalous  glands  may  appear  anywhere 
along  the  line  of  the  original  milk  ridge,  though  they  are  occasionally 
found  elsewhere,  as,  for  instance,  on  the  inner  surface  of  the  thigh. 
They  also  vary  greatly  in  their  development,  their  presence  being  fre- 
quently indicated  merely  by  a  nipple-like  elevation  (hypertheHa). 
Such  accessory  nipples  sometimes  occur  in  the  areolar  area  of  an  other- 
wise normal  gland  and  in  such  cases  may  represent  an  hypertrophy  of 
one  or  more  of  Montgomery's  glands. 


LITERATURE  1 53 

It  is  stated  that  a  slight  and  temporary  enlargement  of  the  gland 
occurs  at  each  premenstrual  period,  but  if  pregnancy  supervenes 
marked  enlargement  occurs  and  a  certain  amount  of  secretion  is  formed, 
this,  however,  not  being  true  milk,  but  a  watery  fluid,  rich  in  proteids 
and  known  as  colostrum.  It  is  only  after  parturition  that  the  secre- 
tion of  milk  begins,  apparently  standing  in  some  relation  to  the  expul- 
sion of  the  fetus.  It  was  formerly  supposed  that  the  correlation  of  the 
activity  of  the  mammary  glands  with  uterine  conditions  was  dependent 
upon  some  nervous  connection,  but  this  has  been  shown  to  be  fallacious 
and  it  seems  more  probable  that  the  stimulus  which  excites  the  gland 
is  chemical  in  its  nature.    There  is  experimental  evidence  that  indicates 


Fig.  91. — Section  through  the  Mammary  Gland  of  an  Embryo  of  25  cm. 
I,  Stroma  of  the  gland. — (From  Nagel,  after  Basch.) 

that  the  growth  of  the  gland  during  pregnancy  is  due  to  a  hormone 
produced  in  the  tissues  of  the  embryo  and  fetus,  this  hormone  inhibiting 
milk  secretion,  while  it  stimulates  the  growth  of  the  gland,  and  on  the 
explusion  of  the  fetus,  the  cause  or  the  inhibition  being  removed,  the 
hypertrophied  gland  starts  to  function.  Although  the  glands  are  nor- 
mally functional  only  in  females,  cases  are  not  unknown  in  which  they 
have  become  well  developed  and  functional  in  males  {gynoecomastia) . 
Furthermore,  a  functional  activity  of  the  glands  normally  occurs  im- 
mediately after  birth,  infants  of  both  sexes  yielding  a  few  drops  of  a 
milky  fluid,  the  so-called  witch-milk  (Hexenmilch),  when  the  glands  are 
subjected  to  pressure. 

LITERATURE 

R.   Bonnet:  "Die  Mammarorgane  im  Lichte  der  Ontogenie  und  Phylogenie," 

Ergebn.  Anat.  und  Entwick.,  11,  1892;  vii,  1898. 
J.  T.  Bowen:  "The  Epitrichial  Layer  of  the  Human  Epidermis,"  Anat.  Anzeiger, 

TV,  1889. 
Brouha:  "Recherches  sur  les  diverses  phases  du  d^veloppement  et  de  I'activite  de  la 

mammelle,"  Arch,  de  Biol.,  xxi,  1905. 
G.  Burckhard:  "Ueber  embryonale  Hypermastie  und  H3^erthelie,"  Anat.  Hefte, 
•  VIII,  1897. 


1 54  LITERATURE 

H.  Eggeling:  "Ueber  ein  wichtiges  Stadium  in  der  Entwicklumgsgeschichte  der 

menschlichen  Brustdriise,"  AnaL  Anzeiger,  xxiv,  1896. 
H.  Head:  "On  Disturbances  of  SensE^tion  with  Special  Reference  to  the  Pain  of 

Visceral  Disease,"  Brain,  xvi,  i892;xvii,  1894;  andxrx,  1896. 
E.  Kallius:  "Ein  Fall  von  Milchleiste  bei  einem   menschlichen  Embryo,"  Anal. 

Hefte,  vni,  1897. 
J.  E.  Lane-Claypon  and  E.  S.  Starling:  "An  Experimental  inquiry  into  the  factors 

which  determine  the  growth  and  activity  of  the  mammary  glands,"  Proc.  Roy. 

Soc.  London,  Ser.  B.y  lxxvii,  1906. 
Hilda    Lustig:  "Zur    Entwicklungsgeschichte    der    menschlichen    Bnistdriise." 

Arch,  fur  mikr.  AnaL,  lxxxvii,  1915. 
T.  Okamura:  "Ueber  die  Entwicklung  des  Nagels  beim  Menschen,"  Archiv  fur 

Dermatol,  und  SyphiloL,  xxv,  1900. 
H.  Schmidt:  "Ueber  normale  Hyperthelie  menschlicher  Embryonen  und  iiber  die 

erste   Anlage  der  menschlichen  Milchdriisen   uberhaupt,"  Morphol.  Arheiten, 

xvii,  1897. 
O.  Schultz:  "Ueber  die  erste  Anlage  des  milchdriisen  Apparates."     Anat.  An- 
zeiger, viii,  1892. 
C.  S.  Sherrington:  "Experiments  in  Examination  of  the  Peripheral  Distribution  of 

the  Fibres  of  the  Posterior  Roots  of  some  Spinal  Nerves,"  Philos.  Trans.  Royal, 

Soc,  CLXxxiv,  1893,  and  cxc,  1898. 
P.  Stohr:  " Entwickelungsgeschichte  des  menschlichen  Wollhaares,"  Anat.  Hefte. 

xxiii,  1903. 
M.    Strahl:    "Die   erste   Entwicklung   der    Mammarorgane   beim    Menschen," 

Verhandl.  Anat.  Gesellsch.,  xii,  1898. 
R.  Zander:  "Die  fruhesten  Stadien  der  Nagelentwicklung  und  ihre  Beziehungen 

zu  den  Digitalnerven,  Arch,  fur  Anat.  und  Physiol.,  Anat.  Ahth.,  1884. 


CHAPTER  VII 

THE  DEVELOPMENT   OF  THE    CONNECTIVE  TISSUES 
AND  SKELETON 

It  has  been  seen  that  the  cells  of  a  very  considerable  portion  of 
the  somatic  and  splanchnic  mesoderm,  as  well  as  of  parts  of  the 
mesodermic  somites,  become  converted  into  mesenchyme.  A 
very  considerable  portion  of  this  becomes  converted  into  what  are 
termed  connective  or  supporting  tissues,  characterized  by  con- 
sisting of  a  non-cellular  matrix  in  which  more  or  less  scattered 
cells  are  embedded.  These  tissues  enter  to  a  greater  or  less  extent 
into  the  formation  of  all  the  organs  of  the  body,  with  the  exception 
of  those  forming  the  central  nervous  system,  and  constitute  a 
network,  which  holds  together  and  supports  the  elements  of  which 
the  organs  are  composed;  in  addition,  they  take  the  form  of 
definite  membranes  (serous  membranes,  fasciae),  cords  (tendons, 
ligaments),  or  solid  masses  (cartilage),  or  form  looser  masses  or 
layers  of  a  somewhat  spongy  texture  (areolar  tissue).  The  inter- 
mediate substance  is  somewhat  varied  in  character,  being  com- 
posed sometimes  of  white, non-branching,  non-elastic  fibers;  some- 
times of  yellow,  branching,  elastic  fibers;  of  white,  branching,  but 
inelastic  fibers  which  form  a  reticulum;  or  of  a  soft  gelatinous 
substance  containing  considerable  quantities  of  mucin,  as  in  the 
tissue  which  constitutes  the  Whartonian  jelly  of  the  umbilical 
cord.  Again,  in  cartilage  the  matrix  is  compact  and  homo- 
geneous, or,  in  other  cases,  more  or  less  fibrous,  passing  over  into 
ordinary  fibrous  tissue,  and,  finally,  in  bone  the  organic  matrix  is 
largely  impregnated  with  salts  of  lime. 

Two  views  exist  as  to  the  mode  of  formation  of  the  matrix, 
some  authors  maintaining  that  in  the  fibrous  tissues  it  is  produced 
by  the  actual  transformation  of  the  mesenchyme  cells  into  fibers, 
while  others  claim  that  it  is  manufactured  by  the  cells  but  does  not 

155 


156  DEVELOPMENT    OF    CONNECTIVE    TISSUE 

directly  represent  the  cells  themselves.  Fibrils  and  material  out 
of  which  fibrils  could  be  formed  have  undoubtedly  been  observed 
in  connective-tissue  cells,  but  whether  or  not  these  are  later  passed 
to  the  exterior  of  the  cell  to  form  a  connective-tissue  fiber  is  not  yet 
certain,  and  on  this  hangs  mainly  the  difference  between  the 
theories.  >^ 

Recently  it  has  been  held  (Mall)  that  the  mesenchyme  of  the 
embryo  is  really  a  syncytium  in  and  from  the  protoplasm^of  which 


a^^^^l^  (ir^('o 


i^'  W^ 


m  r,  ^ 


Fig.  92. — Portion  of  the  Center  of  Ossification  of  the  Parietal  Bone  of  a 

Human  Embryo. 

the  matrix  forms;  if  this  be  correct,  the  distinction  which  the  older 
views  make  between  the  intercellular  and  intracellular  origin  of 
the  matrix  becomes  of  little  importance. 

Bone  differs  from  the  other  varieties  of  connective  tissue  in 
that  it  is  never  a  primary  formation,  but  is  always  developed  either 
in  fibrous  tissue  or  cartilage;  and  according  as  it  is  associated  with 
the  one  or  the  other,  it  is  spoken  of  as  membrane  bone  or  cartilage 
bone.  In  the  development  of  membrane  bone  some  of  the  con- 
nective-tissue cells,  which  in  consequence  become  known  as  osteo- 
blasts, deposit  lime  salts  in  the  matrix  in  the  form  of  bony  spicules 
which  increase  in  size  and  soon  unite  to  form  a  network  (Fig.  92). 
The  trabeculae  of  the  network  continue  to  thicken,  while,  at  the 
same  time,  the  formation  of  spicules  extends  further  out  into  the 
connective-tissue  membrane,  radiating  in  all  directions  from 
the  region  in  which  it  first  developed.    Later  the  connective 


DEVELOPMENT    OF  BONE 


157 


tissue  which  h*es  upon  either  surface  of  the  reticular  plate  of 
bone  thus  produced  condenses  to  form  a  stout  membrane,  th^- 
periosteum,  between  which  and  the  osseous  plate  osteoclasts 
arrange  themselves  in  a  more  or  less  definite  layer  and  deposit 
upon  the  surface  of  the  plate  a  lamella  of  compact  bone.  A 
membrane  bone,  such  as  one  of  the  flat  bones  of  the  skull,  thus 
comes  to  be  composed  of  two 
plates  of  compact  bone,  the  inner 
and  outer  tables,  enclosing  and 
united  to  a  middle  plate  of  spongy 
bone  which  constitutes  the  diploe. 
With  bones  formed  from  car- 
tilage the  process  is  somewhat 
different.  In  the  center  of  the 
cartilage  the  intercellular  matrix 
becomes  increased  so  that  the 
cells  appear  to  be  more  scattered 
and  a  calcareous  deposit  forms 
in  it.  All  around  this  region  of 
calcification  the  cells  arrange 
themselves  in  rows  (Fig.  93)  and 
the  process  of  calcification  ex- 
tends into  the  trabeculae  of  mat- 
rix which  separate  these  rows. 
While  these  processes  have  been   ^^  3^^  months. 

*  c.  Cartilage  trabeculas;    p,  periosteal 

takmg     place      the     mesenchyme     bone;     po,    periosteum;     x,    ossification 

surrounding  the  cartilage  has  be-  eenter.-(5.ymono^/...) 
come  converted  into  a  periosteum  (^<?) ,  similar  to  that  of  membrane 
bone,  and  its  osteoclasts  deposit  a  layer  of  bone  {p)  upon  the  sur- 
face of  the  cartilage.  The  cartilage  cells  now  disappear  from  the 
intervals  between  the  trabeculae  of  calcified  matrix,  which  form 
a  fine  network  into  which  masses  of  mesenchyme  (Fig.  94  pi), 
containing  blood-vessels  and  osteoclasts,  here  and  there  penetrate 
from  the  periosteum,  after  having  broken  through  the  layer  of 
periosteal  bone.  These  masses  absorb  a  portion  of  the  fine 
calcified  network  and  so  transform  it  into  a  coarse  network,  the 


Fig.  93. — Longitudinal  Section  of 
Phalanx  of  a  Finger  of  an  Embryo 


158 


DEVELOPMENT   OF  BONE 


'^^'Hp', 


po 


pi 


meshes  of  which  they  occupy  to  form  the  hone  marrow  (w),  and  the 
osteoclasts  which  they  contain  arrange  themselves  on  the  surface 
of  the  persisting  trabeculse  and  deposit  layers  of  bone  upon  their 
surfaces.  In  the  meantime  the  calcification  of  the  cartilage  matrix 
has  been  extending,  and  as  fast  as  the  network  of  calcified  tra- 

beculae  is  formed  it  is  invaded  by 
the  mesenchyme,  until  finally  the 
cartilage  becomes  entirely  con- 
verted into  a  mass  of  spongy 
bone  enclosed  within  a  layer  of 
more  compact  periosteal  bone. 

As  a  rule,  each  cartilage  bone 
is  developed  from  a  single  center 
of  ossification,  and  when  it  is 
found  that  a  bone  of  the  skull, 
for  instance,  develops  by  several 
centers,  it  is  to  be  regarded  as 
formed  by  the  fusion  of  several 
primarily  distinct  bones,  a  con- 
clusion which  may  generally  be 
confirmed  by  a  comparison  of  the 
bone  in  question  with  its  homo- 
logues  in  the  lower  vertebrates. 
Exceptions  to  this  rule  occur  in 
bones  situated  in  the  median  line 
of  the  body,  these  occasionally  developing  from  two  centers  lying 
one  on  either  side  of  the  median  line,  but  such  centers  are  usually 
to  be  regarded  as  a  double  center  rather  than  as  two  distinct 
centers,  and  are  merely  an  expression  of  the  fundamental  bilater- 
ality  which  exists  even  in  median  structures. 

More  striking  exceptions  are  to  be  found  in  the  long  bones  in 
which  one  or  both  extremities  develop  from  special  centers  which 
give  rise  to  the  epiphyses  (Fig.  95,  ep,  ep'),  the  shaft  or  diaphysis 
{d)  being  formed  from  the  primary  center.  Similar  secondary 
centers  appear  in  marked  prominences  on  bones  to  which  power- 
ful muscles  are  attached  (Fig.  95,  a  and  h),  but  these,  as  well  as 


3. 


Fig.  94. — The  Ossification  Center 
OF  Fig.  92  More  Highly  Magnified. 
c.  Ossifying  trabeculae;  cc,  cavity  of 
cartilage  network;  m,  marrow  cells;  p, 
periosteal  bone;  pi,  irruption  of  peri- 
osteal tissue ;  po,  periosteum. — Szymo- 
nowicz.) 


DEVELOPMENT    OF   BONE 


159 


the  epiphysial  centers,  can  readily  be  recognized  as  secondary 
from  the  fact  that  they  do  not  appear  until  much  later  than  the_ 
primary  centers  of  the  bones  to  which  they  belong.     These  sec- 
ondary centers  give  the  necessary  firmness  required  for  articular 
surfaces  and  for  the  attachment   of  muscles 
and,  at  the  same  time,  make  provision  for  the 
growth  in  length  of  the  bone,  since  a  plate  of 
cartilage  always  intervenes  between  the  epi- 
physes and  the  diaphysis.     This  cartilage  con- 
tinues to  be  transformed  into  bone  on  both 
its   surfaces   by  the  extension  of  both  the 
epiphysial  and  diaphysial  ossification  into  it, 
and,  at  the  same  time,  it  grows  in  thickness 
with  equal  rapidity  until  the  bone  reaches  its 
required  length,  whereupon   the  rapidity  of 
the   growth  of   the  cartilage  diminishes  anli 
it  gradually  becomes  completely  ossified,  unit- 
ing together  the  epiphysis  and  diaphysis. 

The  growth  in  thickness  of  the  long 
bones  is,  however,  an  entirely  different 
process,  and  is  due  to  the  formation  of  new 
layers  of  periosteal  bone  on  the  outside  of 
those  already  present.  But  in  connection 
with  this  process  an  absorption  of  bone  also 
takes  place.  A  section  through  the  middle 
of  the  shaft  of  a  humerus,  for  example,  at  an 
early  stage  of  development  would  show  a  peri- 
pheral zone  of  compact  bone  surrounding  a 
core  of  spongy  bone,  the  meshes  of  the  latter 
being  occupied  by  the  marrow  tissue.  A 
similar  section  of  an  adult  bone,  on  the  other  hand,  would  show 
only  the  peripheral  compact  bone,  much  thicker  than  before  and 
enclosing  a  large  marrow  cavity  in  which  no  trace  of  spongy  bone 
might  remain.  The  difference  depends  on  the  fact  that  as  the 
periosteal  bone  increases  in  thickness,  there  is  a  gradual  absorp- 
tion of  the  spongy  bone  and  also  of  the  earlier  layers  of  periosteal 


Fig.  95. — The  Ossi- 
fication Centers  of 
THE  Femur. 

a,  and  h.  Secondary- 
centers  for  the  great 
and  lesser  trochanters; 
d,  diaphysis;  ep,  upper 
and  ep',  lower  epiphysis. 
~{Testut.) 


l6o  DEVELOPMENT    OF   BONE 

bone,  this  absorption  being  carried  on  by  large  multinucleated 
cells,  termed  osteoclasts^  derived  from  the  marrow  mesenchyme. 
By  their  action  the  bone  is  enabled  to  reach  its  requisite  diameter 
and  strength,  without  becorning  an  almost  solid  and  unwieldy 
mass  of  compact  bone. 

It  has  recently  been  claimed  (Arey)  that  the  evidence  showing 
that  the  osteoclasts  are  the  effective  agents  in  bone  absorption  is 
entirely  inadequate.  Instead  of  being  active  structures  they  are  held 
to  be  produced  by  the  fusion  of  degenerate  and  worn-out  osteoclasts. 


Fig.  96. — A,  Transverse  Section  of  the  Femur  of  a  Pig  Killed  after 
Having  been  fed  with  Madder  for  Four  Weeks;  B,  the  Same  of  a  Pig  Killed 
Two  Months  after  the  Cessation  of  the  Madder  Feeding. 
The  heavy  black  line  represents  the  portion  of  bone  stained  by  the  madder. — {After 

Flour  ens.) 

During  the  ossification  of  the  cartilaginous  trabeculae  osteo- 
clasts become  enclosed  by  the  bony  substance,  the  cavities  in 
which  they  lie  forming  the  lacuncB  and  processes  radiating  out 
from  them,  the  canaliculi,  so  characteristic  of  bone  tissue.  In 
the  growth  of  periosteal  bone  not  only  do  osteoclasts  become 
enclosed,  but  blood-vessels  also,  the  Haversian  canals  being  formed 
in  this  way,  and  lamellae  of  bone  are  deposited  around  these  by 
the  enclosed  osteoclasts  to  form  Haversian  systems. 

That  the  absorption  of  periosteal  bone  takes  place  during  growth 
can  be  demonstrated  by  taking  advantage  of  the  fact  that  the  coloring 
substance  madder,  when  consumed  with  food,  tinges  the  bone  being 
formed  at  the  time  a  distinct  red.  In  pigs  fed  with  madder  for  a  time 
and  thcii  killed  a  section  of  the  femur  shows  a  superficial  band  of  red 
bone  (Fig.  96  ^),  but  if  the  animals  be  allowed  to  live  for  one  or  two 
months  after  the  cessation  of  the  madder  feeding,  the  red  band  will  be 
found  to  be  covered  by  a  layer  of  white  bone  varying  in  thickness 


DEVELOPMENT  OF  THE  SKELETON 


l6l 


according  to  the  interval  elapsed  since  the  cessation  of  feeding  (Fig. 
96  B) ;  and  if  this  interval  amount  to  four  months,  it  will  be  found  that 
the  thickness  of  the  uncolored  bone  between  the  red  bone  and  the- 
marrow  cavity  will  have  greatly  diminshed  (Flourens). 

The  Development  of  the  Skeleton. — Embryologically  con- 
sidered, the  skeleton  is  composed  of  two  portions,  the  axial  skeleton 


isa 


Sc/i^^J^ 


tfca 


Pig.  97. — Frontal  Section  through  Mesodermic  Somites  of  a  Calf  Embryo. 

isa.  Intersegmental  artery;  my,  myotome;  n,  central    nervous  system;  nc,  notochord; 

sea  and  scp,  anterior  and  posterior  portions  of  sclerotomes. 

consisting  of  the  skull,  the  vertebrae,  ribs,  and  sternum,  developing 
from  the  sclerotomes  of  the  mesodermal  somites,  and  the  appen- 
dicular skeleton,  which  includes  the  pectoral  and  pelvic  girdles  and 
the  bones  of  the  limbs,  and  which  arises  from  the  mesenchyme  of 
the  somatic  mesoderm.  It  will  be  convenient  to  consider  first 
the  development  of  the  axial  skeleton,  and  of  this  the  differen- 
tiation of  the  vertebral  column  and  ribs  may  first  be  discussed. 
11 


102 


DEVELOPMENT  OF  THE  VERTEBRA 


The  Development  of  the  Vertebrae  and  Ribs. — The  mesen- 
chyme formed  from  the  sclerotome  of  each  mesodermic  somite 
grows  inward  toward  the  median  line  and  forms  a  mass  lying 
between  the  notochord  and  the  myotome,  separated  from  the 
similar  mass  in  front  and  behind  by  some  loose  tissue  in  which  lies 
an  intersegmental  artery.  Toward  the  end  of  the  third  week  of 
development  the  cells  of  the  posterior  portion  of  each  sclerotome 
condense  to  a  tissue  more  compact  than  that  of  the  anterior 

portion  (Fig.  97),  and  a  little 
later  the  two  portions  become 
separated  by  a  cleft.  At  about 
the  same  time  the  posterior  por- 
tion sends  a  process  medially,  to 
enclose  the  notochord  by  uniting 
with  a  corresponding  process 
from  the  sclerotome  of  the  other 
side  and  it  ^Iso  sends  a  prolong- 
ation dorsally  between  the  myo- 
tome and  the  spinal  cord  to 
form  the  vertebral  arch,  and  a 
third  process  laterally  and 
ventrally  along  the  distal  border 
of  the  myotome  to  form  a  costal 
process  (Fig.  98).  The  looser 
tissue  of  the  anterior  half  of 
the  sclerotome  also  grows  medially  to  surround  the  noto- 
chord, filling  up  the  intervals  between  successive  denser  portions, 
and  it  forms  too  a  membrane  extending  between  successive  verte- 
bral arches.  Later  that  tissue  surrounding  the  notochord  which  is 
derived  from  the  anterior  half  of  the  sclerotome,  associates  itself 
with  the  posterior  portion  of  the  preceding  sclerotome  to  form 
what  will  later  be  a  vertebra,  the  tissue  occupying  and  adjacent 
to  the  line  of  division  between  the  anterior  and  posterior  por- 
tions of  the  sclerotomes  condensing  to  form  intervertebral  fibro- 
cartilages.  Consequently  each  vertebra  is  formed  by  parts  from 
two  sclerotomes,  the  original  intersegmental  artery  passes  over 


Fig.      98.  —  Transverse      Section 

THROUGH    the    INTERVERTEBRAL  PlATE 

OF  THE  First  Cervical  Vertebra  of 
A  Calf  Embryo  of  8.8  mm. 

bc^.  Intervertebral  plate;  m*,  fourth 
myotome;  s,  hypochordal  bar;  XI, 
spinal  accessory  nerve. — (Froriep.) 


DEVELOPMENT  OF  THE  VERTEBRA 


163 


the  body  of  a  vertebra,  and  the  vertebrae  themselves  alternate 
with  the  myotomes  (Fig.  99).  With  this  differentiation  the  first 
or  blastemic  stage  of  the  development  of  the  vertebrae  closes. 

In  the  second  or  cartilaginous  stage,  portions  of  the  sclero- 
tomic  mesenchyme  become  transformed  into  cartilage.  In  the 
posterior  portion  of  each  vertebral  body,  that  is  to  say  in  the 
portion  formed  from  the  anterior  halves  of  the  more  posterior  of 
the  two  pairs  of  sclerotomes  entering  into  its  formation,  two 
centers  of  chondr'fication  appear,  one  on  each  side  of  the  median 


Veri  So^ 


Mes  Som 

Pig.  99. — Diagram  to  show  the  Relation  of  the  Vertebra   to  the   Meso- 

DERMic  Somites. 
ID,  Intervetebral  disk;  LS,  ligamentum  flavum;  NA,  neural  arch,    V,    vertebra 

line,  and  these  eventually  unite  to  form  a  single  cartilaginous  body, 
the  chondrification  probably  also  extending  to  some  extent  into  the 
denser  anterior  portion  of  the  body.  A  center  also  appears  in 
each  half  of  the  vertebral  arch  and  in  each  costal  process,  the 
cartilages  formed  in  the  costal  processes  of  the  anterior  cervical 
region  uniting  across  the  median  line  below  the  notochord,  to  form 
what  has  been  termed  a  hypochordal  bar  (Figs.  98  and  100).  These 
bars  are  for  the  most  part  but  transitory,  recalling  structures 


164  DEVELOPMENT    OF   THE  VERTEBRAE 

occurring  in  the  lower  vertebrates;  in  the  mammalia  they  de- 
generate before  the  close  of  the  cartilaginous  stage  of  development, 
except  in  the  case  of  the  atlas,  whose  development  will  be  de- 
scribed later.  As  development  proceeds  the  cartilages  of  the 
vertebral  arches  and  costal  processes  increase  in  length  and  come 
into  contact  with  the  cartilaginous  bodies,  with  which  they 
eventually  fuse,  and  from  the  vertebral  arches  processes  grow  out 
which  represent  the  future  transverse  and  articular  processes. 

The  fusion  of  the  cartilage  of  the  costal  process  with  the  body 
of  the  vertebra  does  not,  however,  persist.  Later  a  solution  of  the 
junction  occurs  and  the  process  becomes  a  rib  cartilage,  the  mesen- 
chyme surrounding  the  area  of  solution  forming  the  costo-vertebral 
ligaments.  At  first  the  rib  cartilage  is  separated  by  a  distinct 
interval  from  the  transverse  process  of  the  vertebral  arch,  but 
later  it  develops  a  process,  the  tubercle,  which  bridges  the  gap  and 
forms  an  articulation  with  the  transverse  process. 

The  mesenchyme  which  extends  between  successive  vertebral 
arches  does  not  chondrify,  but  later  becomes  transformed  into  the 
interspinous  ligaments  and  the  ligamenta  flava,  while  the  anterior 
and  posterior  longitudinal  ligaments  are  formed  from  unchondri- 
fied  portions  of  the  tissue  surrounding  the  vertebral  bodies. 

As  was  pointed  out,  the  mesenchyme  in  the  region  of  the  cleft 
separating  the  anterior  and  posterior  portions  of  a  sclerotome  be- 
comes an  intervertebral  fibrocartilage,  and,  as  the  cartilaginous 
bodies  develop,  the  portions  of  the  notochord  enclosed  by  them 
become  constricted,  while  at  the  same  time  the  portions  in  the 
intervertebral  regions  increase  in  size.  Finally  the  notochord  dis- 
appears from  the  vertebral  regions,  although  a  canal,  represent- 
ing its  former  position,  traverses  each  body  for  a  considerable 
time,  but  in  the  intervertebral  regions  it  persists  as  relatively  large 
flat  disks  forming  the  pulpy  nuclei  of  the  fibrocartilages. 

The  mode  of  development  described  above  applies  to  the  great 
majority  of  the  vertebrae,  but  some  departures  from  it  occur,  and 
these  may  be  conveniently  considered  before  passing  on  to  an 
account  of  the  ossification  of  the  cartilages.  The  variations  affect 
principally  the  extremes  of  the  series.     Thus  the  posterior  verte- 


DEVELOPMENT  OF  THE  VERTEBRA 


165 


brae  present  a  reduction  of  the  vertebral  arches,  those  of  the  last 
sacral  vertebrae  being  but  feebly  developed,  while  in  the  coccygeal 
vertebrae  they  are  indicated  only  in  the  first.  In  the  first  cervical 
vertebra,  the  atlas,  the  reverse  is  the  case,  for  the  entire  adult 
vertebra  is  formed  from  the  posterior  portion  of  a  sclerotome,  its 
lateral  masses  and  posterior  arch  being  the  vertebral  arches,  while 
its  anterior  arch  is  the  hypochordal  bar,  which  persists  in  this 


Kc^ 

^Ic- 

\ 

Kc^ 

^Ic^ 

m\ 

%'4 

ch 

Fig.  100. — Longitudinal  Section  through  the  Occipital  Region  and  Upper 

Cervical  Vertebra  of  a  Calf  Embryo  of  18.5  mm. 

bas,    Basilar  artery;   ch,  'notochord;   Kc^~*,  vertebral  centra;   lc^~*.  intervertebral 

disks;  occ,  basioccipital;  Sc^~^,  hypochordal  bars. — (Froriep.) 


vertebra  only.  A  well-developed  centrum  is  also  formed,  however 
(Fig.  100),  but  it  does  not  unite  with  the  parts  derived  from  the 
preceding  sclerotome,  but  during  its  ossification  unites  with  the 
centrum  of  the  epistropheus  (axis),  forming  the  odontoid  process 
of  that  vertebra.  The  epistropheus  consequently  is  formed  by 
one  and  a  half  sclerotomes,  while  but  half  a  one  constitutes  the 
atlas. 

The  extent  to  which  the  ribs  are  developed  in  connection  with 


1 66  DEVELOPMENT  OF  THE  VERTEBRAE 

the  various  vertebrae  also  varies  considerably.  Throughout  the 
cervical  region  they  are  short,  the  upper  five  or  six  being  no 
longer  than  the  transverse  processes,  with  the  tips  of  which  their 
extremities  unite  at  an  early  stage.  In  the  upper  five  or  six 
vertebrae  a  relatively  large  interval  persists  between  the  rib  and 
the  transverse  process,  forming  the  costo-transverse  foramen, 
through  which  the  vertebral  vessels  pass,  but  in  the  seventh  verte- 
bra the  fusion  is  more  extensive  and  the  foramen  is  very  small 
and  hardly  noticeable.  In  the  thoracic  region  the  ribs  reach  their 
greatest  development,  the  upper  eight  or  nine  extending  almost 
to  the  mid-ventral  line,  where  their  extremities  unite  with  a 
longitudinal  cartilaginous  bar  from  which  the  sternum  develops 
(see  p.  1 68).  The  lower  three  or  four  thoracic  ribs  are  success- 
ively shorter,  however,  and  lead  to  the  condition  found  in  the 
lumbar  vertebrae,  where  they  are  again  greatly  reduced  and  firmly 
united  with  the  transverse  processes,  the  union  being  so  close 
that  only  the  tips  of  the  latter  can  be  distinguished,  forming  what 
are  known  as  the  accessory  tubercles.  In  the  sacral  region  the 
ribs  are  reduced  to  short  flat  plates,  which  unite  together  to  form 
the  lateral  masses  of  the  sacrum,  and,  finally,  in  the  coccygeal  re- 
gion the  blastemic  costal  processes  of  the  first  vertebra  unite  with 
the  transverse  processes  to  form  the  transverse  processes  of  the 
adult  vertebra,  but  no  indications  of  them  are  to  be  found  in  the 
other  vertebrae  beyond  the  blastemic  stage. 

The  third  stage  in  the  development  of  the  axial  skeleton  begins 
with  the  ossification  of  the  cartilages,  and  in  each  vertebra  there 
are  typically  as  many  primary  centers  of  ossification  as  there 
were  originally  cartilages.  Thus,  to  take  a  thoracic  vertebra  as 
a  type,  a  center  appears  in  each  half  of  each  vertebral  arch  at  the 
base  of  the  transverse  process  and  gradually  extends  to  form  the 
bony  lamina,  pedicle,  and  the  greater  portion  of  the  transverse 
and  spinous  processes;  a  single  center  gives  rise  to  the  body  of 
the  vertebra;  and  each  rib  ossifies  from  a  single  center.  These 
various  centers  appear  early  in  embryonic  life,  but  the  complete 
transformation  of  the  cartilages  into  bone  does  not  occur  until 
some  time  after  birth,  each  vertebra  at  that  period  consisting  of 


DEVELOPMENT  OF  THE  VERTEBRA  1 67 

three  parts,  a  body  and  two  halves  of  an  arch,  separated  by  unos- 
sified  cartilage  (Fig.  loi,  ^) .  At  about  puberty  secondary  centerj^ 
make  their  appearance;  one  appears  in  the  cartilage  which  still 
covers  the  anterior  and  posterior  surfaces  of  the  vertebral  body, 
producing  disks  of  bone  in  these  situations  (Fig.  loi,  B,  en  and  el), 
another  appears  at  the  tip  of  each  spinous  and  transverse  process 
(Fig.  loi,  ^),  and  in  the  lumbar  vertebrae  others  appear  at  the  tips 
of  the  articulating  processes.  The  epiphyses  so  formed  remain 
separate  until  growth  is  completed  and  between  the  sixteenth  and 
twenty-fifth  years  unite  with  the  bones  formed  from  the  primary 
centers,  which  hav^  fused  by  this  time  to  form  a  single  vertebra. 
Each  rib  ossifies  from  a  single  primary  center  situated  near 
the  angle,  secondary  centers  appearing  for  the  capitulum  and 
tuberosity. 


Fig.  ioi. — A,  A  Vertebra  at  Birth;  B,  Lumbar  Vertebra  showing  Secondary 

Centers  of  Ossification. 
a.  Center  for  the  articular  process;  c,  body;  el,  lower  epiphysial  plate;  en,  upper 
epiphysial  plate;  na,  vertebral  arch;  s,  center  for  spinous  process;  t,  center  for  trans- 
verse process. — (Sappey.) 

In  some  of  the  vertebrae  modifications  of  this  typical  mode  of 
ossification  occur.  Thus,  in  the  upper  five  cervical  vertebrae  the 
centers  for  the  rudimentary  ribs  are  suppressed,  ossification  ex- 
tending into  them  from  the  vertebral  arch  centers,  and  a  similar 
suppression  of  the  costal  centers  occurs  in  the  lower  lumbar  verte- 
brae, the  first  only  developing  a  separate  rib  center.  Furthermore, 
in  the  atlas  a  double  center  appears  in  the  persisting  hypochordal 
bar,  and  the  body  which  corresponds  to  the  atlas,  after  developing 
the  terminal  epiphysial  disks,  fuses  with  the  body  of  the  epis- 
tropheus (axis)  to  form  its  odontoid  process,  this  vertebra  conse- 
quently possessing,  in  addition  to  the  typical  centers,  one  (double) 


i68 


DEVELOPMENT    OF    THE    STERNUM 


other  primary  and  two  secondary  centers.  In  the  sacral  region  the 
typical  centers  appear  in  all  five  vertebrae,  with  the  exception  of 
rib  centers  for  the  last  one  or  two  (Fig.  102) ,  and  two  additional 
secondary  centers  give  rise  to  plate-like  epiphyses  on  each  side, 
the  upper  plates  forming  the  articular  surface  for  the  ilium.  At 
about  the  twenty-fifth  year  all  the  sacral  vertebrae  unite  to  form 
a  single  bone,  and  a  similar  fusion  occurs  also  in  the  rudimentary 
vertebrae  of  the  coccyx. 

The  majority  of  the  anomalies  seen  in  the  vertebral  column  are  due 
to  the  imperfect  development  of  one  or  more  cartilages  or  of  the  centers 
of  ossification.     Thus,  a  failure  of  an  arch  to  unite  with  the  body  or 


Fig.   102. — A,  Upper  Surface  of  the  First  Sacral  Vertebra,  and  B,  Ventral 
View  of  the  Sacrum  showing  Primary  Centers  of  Ossification,     y 
c.  Body;  na,  vertebral  arch;  r,  rib  center. — (Sappey.) 

even  the  complete  absence  of  an  arch  or  half  an  arch  may  occur,  and  in 
cases  of  spina  bifida  the  two  halves  of  the  arches  fail  to  unite  dorsally. 
Occasionally  the  two  parts  of  the  double  cartilaginous  center  for  the 
body  fail  to  unite,  a  double  body  resulting;  or  one  of  the  two  parts 
may  entirely  fail,  the  result  being  the  formation  of  only  one-half  of 
the  body  of  the  vertebra.  Other  anomalies  result  from  the  excessive 
development  of  parts.  Thus,  the  rib  of  the  seventh  cervical  vertebra 
may  sometines  remain  distinct  and  be  almost  long  enough  to  reach  the 
sternum,  and  the  first  lumbar  rib  may  also  fail  to  unite  with  its  vertebra. 
On  the  other  hand,  the  first  thoracic  rib  is  occasionally  found  to  be 
imperfect. 

The  Development  of  the  Sternum. — ^Longitudinal  cartilagin- 
ous bars,  with  which  the  ventral  ends  of  the  anterior  eight  or 


DEVELOPMENT    OF   THE    STERNUM 


169 


nine  cartilaginous  thoracic  ribs  unite,  represent  the  future  sternum. 
At  an  early  period  the  two  bars  come  into  contact  anteriorly  and^ 
fuse  together  (Fig.  103),  and  at  this  anterior  end  two  usually 
indistinctly  separated  masses  of  cartilage  are  to  be  observed  at 
the  vicinity  of  the  points  where  the  ventral  ends  of  the  carti- 
laginous clavicles  articulate.  These  are  the  episternal  cartilages 
(em),  which  later  normally  unite  with  the  longitudinal  bars  and 


Fig.   103. — Formation  of  the  Sternum  in  an  Embryo  of  about  3  cm. 
el,  Clavicle;  em,  episternal  cartilage. — (Ruge.) 


form  part  of  the  manubrium  sterni,  though  occasionally  they 
persistand  ossify  to  form  the  ossa  suprasternalia.  The  fusion 
of  the  longitudinal  bars  gradually  extends  backward  until  a 
single  elongated  plate  of  cartilage  results,  with  which  the  seven 
anterior  ribs  are  united,  one  or  two  of  the  more  posterior  ribs 
which  originally  were  connected  with  each  bar  having  separated. 
The  portions  of  the  bars  separated  from  these  posterior  ribs  con- 
stitute the  xiphoid  process. 

The  ossification  of  the  sternum  (Fig.  104)  partakes  to  a  certain 


170 


DEVELOPMENT    OF    THE    STERNUM 


extent  of  the  original  bilateral  origin  of  the  cartilage,  but  also 
shows  marked  indications  of  a  segmental  arrangement,  the  centers 
tending  to  correspond  with  the  ribs  that  unite  with  the  sternum. 
In  the  portion  of  the  cartilage  which  lies  below  the  junction  of 
the  third  costal  cartilages  a  series  of  two  or  more  pairs  of  centers 
appears  just  about  birth,  each  pair  being  situated  opposite  an 
intercostal  space.  Later  the  centers  of  each  pair  fuse  and  the 
single  centers  so  formed,  extending  through  the  cartilage,  eventu- 
ally unite   to  form  the  greater  part  of  the  body  of  the  bone. 


Pig.  104. — Sternum       of 
New-born    Child,    showing 
Centers  of  Ossification. 
/  to  VII.  Costal  cartilages. — 
(Gegenbaur.) 


Fig.  105. — Reconstruction  of  the  Chondro- 
CRANiuM  of  an  Embryo  of  14  mm. 
as,  Alisphenoid;  bo,  basioccipital;  bs,  basi- 
sphenoid;  eo,  exoccipital;  m,  Meckel's  cartilage; 
OS,  orbitosphenoid;  p,  periotic;  ps,  presphenoid; 
so,  sella  turcia;  s,  supraoccipital. — (Levi.) 


Above  the  attachment  of  the  third  ribs,  however,  two  single 
centers  appear,  the  lower  of  which  unites  with  the  more  posterior 
centers  to  form  the  upper  part  of  the  body,  while  the  uppermost 
center  gives  rise  to  the  manubrium,  which  frequently  persists  as 
a  distinct  bone  united  to  the  body  by  a  hinge-joint. 

It  has  been  generally  supposed  that  the  longitudinal  cartilaginous 
bars  were  formed  by  the  fusion  of  the  upturned  ventral  ends  of  the 
first  eight  ribs,  so  that  each  of  the  sternal  segments  (sternebrae)  might 
be  regarded  as  the  fused  ventral  epiphyses  of  a  pair  of  ribs.  Recent 
observations  on  pig  embryos  indicate,  however,  that  the  cartilaginous 


DEVELOPMENT   OF  THE  SKULL  171 

bars  are  formed  independently  of  the  ribs  (Hanson),  these  latter  unit- 
ing with  them  only  after  they  are  formed.  The  segmental  arrange- 
ment of  the  ossification  centres  seems,  nevertheless,  to  be  determined, 
by  the  association  of  the  ribs  with  the  bars. 

A  failure  of  the  cartilaginous  bars  to  fuse  produces  the  condition 
known  as  cleft  sternum,  or  if  the  failure  to  fuse  affects  only  a  portion  of 
the  bars  there  results  a  perforated  sternum.  A  perforation  or  notch- 
ing of  the  xiphoid  cartilage  is  of  frequent  occurrence  owing  to  this 
being  the  region  where  the  fusion  of  the  bars  takes  place  last. 

The  suprasternal  bones  are  the  rudiments  of  a  bone  or  cartilage,  the 
omosternum,  situated  in  front  of  the  manubrium  in  many  of  the  lower 
mammalia.  It  furnishes  the  articular  surfaces  for  the  clavicles  and  is 
possibly  formed  by  a  fusion  of  the  ventral  ends  of  the  cartilages  which 
represent  those  bones;  hence  its  appearance  as  a  pair  of  bones  in  the 
rudimentary  condition. 

The  Development  of  the  Skull. — In  its  earliest  stages  the 
human  skull  is  represented  by  a  continuous  mass  of  mesench3ane 
which  invests  the  anterior  portion  of  the  notochord  and  extends 
forward  beyond  its  extremity  into  the  nasal  region,  forming  a  core 
for  the  nasal  process  (see  p.  loi).  From  each  side  of  this  basal 
mass  a  wing  projects  dorsally  to  enclose  the  anterior  portion  of 
the  medullary  canal  which  will  later  become  the  cerebral  part  of 
the  central  nervous  system.  No  indications  of  a  segmental 
origin  are  to  be  found  in  this  mesenchyme;  as  stated,  it  is  a  con- 
tinuous mass,  and  this  is  likewise  true  of  the  cartilage  which  later 
develops  in  it. 

The  chondrification  occurs  first  along  the  median  line  in  what 
will  be  the  occipital  and  sphenoidal  regions  of  the  skull  (Fig.  105) 
and  thence  gradually  extends  forward  into  the  ethmoidal  region 
and  to  a  certain  extent  dorsally  at  the  sides  and  behind  into  the 
regions  later  occupied  by  the  wings  of  the  sphenoid  {as  and  os) 
and  the  squamous  portion  of  the  occipital  {s).  No  cartilage  de- 
velops, however,  in  the  rest  of  the^ides  or  in  the  roof  of  the  skull, 
but  the  mesenchyme  of  these  regions  becomes  converted  into  a 
dense  membrane  of  connective  tissue.  While  the  chondrifica- 
tion is  proceeding  in  the  regions  mentioned,  the  mesenchyme  which 
encloses  the  internal  ear  becomes  converted  into  cartilage,  form- 
ing a  mass,  the  periotic  capsule  (Fig.  105,  p),  wedged  in  on  either 
side  between  the  occipital  and  sphenoidal  regions,  with  which  it 


172 


DEVELOPMENT   OF   THE    SKULL 


eventually  unites  to  form  a  continuous  chondrocranium,  perfo- 
rated by  foramina  for  the  passage  of  nerves  and  vessels. 

The  posterior  part  of  the 
basilar  portion  of  the  occipital 
cartilage  presents  certain  pecu- 
liarities of  development.  In  calf 
embryos  there  are  in  this  region,  in 
very  early  stages,  four  separate 
condensations  of  mesoderm  cor- 
responding to  as  many  mesodermic 
somites  and  to  the  three  roots  of 
the  hypoglossal  nerve  together 
with  the  first  cervical  or  subocci- 
pital nerve  (Froriep)  (Fig.  106). 
These  mesenchymal  masses  in 
their  general  characters  and  rela- 
tions resemble  vertebral  bodies, 
and  there  are  good  reasons  for 
believing  that  they  represent  four 
vertebrae  which,  in  later  stages, 
are  taken  up  into  the  skull  re- 
gion and  fuse  with  the  primitive 
chondrocranium.  In  the  human 
embryo  they  are  less  distinct 
than  in  lower  mammals,  but  since 
a  three-rooted  hypoglossal  and 
Fig.  106.— Frontal  Section  Suboccipital  nerve  also  occur  in 
THROUGH  THE  OCCIPITAL  AND  UPPER    ^^^  ^^  ^    probablc  that  thc  cor- 

Cervical   Regions   of  a  Calf  Em-  ^ 

responding  vertebrae  are  also  rep- 
resented. Indeed,  confirmation 
of  their  existence  may  be  found  in 
the  fact  that  during  the  cartilag- 
,  ,        ,      ,  .    inous  stage  of  the  skull  the  hypo- 

01-3,    roots   of    hypoglossal   nerve;    vj,  .  .    .  .-^ 

jugular  vein;  x  and  xi,  vagus  and    glossal  foramiua  are  divided  into 

spinal  accessory  ner^es.-iFroriep.)        ^^^^^  portions  by  tWO  CartilagiuOUS 

partitions  which  separates  the  three  roots  of  the  hypoglossal  nerve. 


BRYO    of    8.7    MM. 

ai  and  at^  Intervertebral  arteries; 
bc^,  first  cervical  intervertebral  plate; 
bo,  suboccipital  intervertebral  plate; 
f:»-2,  cervical  nerves;  ch,  notochord; 
K,  vertebral  centrum;  wi-»,  occipital 
myotomes;   w*-«,  cervical  myotomes; 


DEVELOPMENT    OF    THE    SKULL 


173 


It  seems  certain  from  the  evidence  derived  from  embryology  and 
comparative  anatomy  that  the  human  skull  is  composed  of  a 
primitive  unsegmental  chondrocranium  plus  four  vertebrae,  the 
latter  being  added  to  and  incorporated  with  the  occipital  portion 
of  the  chondrocranium. 

Emphasis  must  be  laid  upon  the  fact  that  the  cartilaginous 
portion  of  the  skull  forms  only  the  base  and  lower  portions  of  the 
sides  of  the  cranium,  its  entire  roof,  as  well  as  the  face  region, 
showing  no  indication  of  cartilage,  the  mesenchyme  in  these 
regions  being  converted  into 
fibrous  connective  tissue, 
which,  especially  in  the  cranial 
region,  assumes  the  form  of 
a  dense  membrane. 

But  in  addition  to  the 
chondrocranium  and  the  ver- 
tebrae incorporated  with  it, 
other  cartilaginous  elements 
enter  into  the  composition  of 
the  skull.  The  mesenchyme 
which  occupies  the  axis  of 
each  branchial  arch  undergoes  more  or  less  complete  chon- 
drification,  cartilaginous  bars  being  so  formed,  certain  of  which 
enter  into  very  close  relations  with  the  skull.  It  has  been  seen 
(p.  94)  that  each  half  of  the  first  arch  gives  rise  to  a  maxillary 
process  which  grows  forward  and  ventrally  to  form  the  anterior 
boundary  of  the  mouth,  while  the  remaining  portion  of  the 
arch  forms  the  mandibular  process.  The  whole  of  the  axis  of  the 
mandibular  process  becomes  chondrified,  forming  a  rod  known  as 
Meckel's  cartilage,  and  this,  at  its  dorsal  end,  comes  into  relation 
with  the  periotic  capsule,  as  does  also  the  dorsal  end  of  the  carti- 
lage of  the  second  arch.  In  the  remaining  three  arches  cartilage 
forms  only  in  the  ventral  portions,  so  that  their  rods  do  not  come 
into  relation  with  the  skull,  though  it  will  be  convenient  to  consider 
their  further  history  together  with  that  of  the  other  branchial  arch 


Fig.    107. — Diagram    showing   the  Five 
Branchial  Cartilages,  /  to  V. 
At,    Atlas;    Ax,    epistropheus;    3,    third 
cervical  vertebra. 


174 


DEVELOPMENT   OF   THE   SKULL 


cartilages.     The  arrangement  of  these  cartilages  is  shown  dia- 
grammatically  in  Fig.  107. 

By  the  ossification  of  these  various  parts  three  categories 
of  bones  are  formed:  (i)  cartilage  bones  formed  in  the 
chondrocranium,  (2)  membrane  bones,  and  (3)  cartilage  bones 
developing  from  the  cartilages  of  the  branchial  arches.  The  bones 
belonging  to  each  of  these  categories  are  primarily  quite  distinct 

from  one  another  and  from  those 
of  the  other  groups,  but  in  the 
human  skull  a  very  considerable 
amount  of  fusion  of  the  primary 
bones  takes  place,  and  elements 
belonging  to  two  or  even  to  all 
three  categories  may  unite  to 
form  a  single  bone  of  the  adult 
skull.  In  a  certain  region  of  the 
chondrocranium  also  and  in  one 
of  the  branchial  arches  the 
original  cartilage  bone  becomes 
ensheathed  by  membrane  bone 
and  eventually  disappears  com- 
pletely, so  that  the  adult  bone, 
-Occipital  Bone  of  a  Fetus  although  represented  by  a  carti- 
AT  Term.  lage,  is  really  a  membrane  bone. 

bo,  Basioccipital;  eo,  exoccipital;  ip,        \     j    •     ^       j    a.v,'  t_ 

interparietal;  so,  supraoccipitai.  And,  mdecd,  this  process  has  pro- 
ceeded so  far  in  certain  portions 
of  the  branchial  arch  skeleton  that  the  original  cartilaginous 
representatives  are  no  longer  developed,  but  the  bones  are 
deposited  directly  in  connective  tissue.  These  various  modi- 
fications interfere  greatly  with  the  precise  application  to  the 
human  skull  of  the  classification  of  bones  into  the  three  categories 
given  above,  and  indeed  the  true  significance  of  certain  of  the 
skull  bones  can  only  be  perceived  by  comparative  studies. 
Nevertheless  it  seems  advisable  to  retain  the  classification,  indicat- 
ing, where  necessary,  the  confusion  of  bones  of  the  various 
categories. 


Fig.  108. 


OSSIFICATION    OF   THE    CHONDROCRANIUM  1 75 

The  Ossification  of  the  Chondrocranium. — The  ossification  of 
the  cartilage  of  the  occipital  region  results  in  the  formation  of 
four  distinct  bones  which  even  at  birth  are  separated  from  onF 
another  by  bands  of  cartilage.  The  portion  of  cartilage  lying  in 
front  of  the  foramen  magnum  ossifies  to  form  a  basioccipital 
bone  (Fig.  io8,  bo),  the  portions  on  either  side  of  this  give  rise  to 
the  two  exoccipitals  (eo),  which  bear  the  condyles,  and  the  por- 
tion above  the  foramen  produces  a  supraoccipital  {so) ,  which  repre- 
sents the  part  of  the  squamous  portion  of  the  adult  bone  lying 
below  the  superior  nuchal  line.  All  that  portion  of  the  bone 
which  lies  above  that  line  is  composed  of  membrane  bone  which 


Fig.  109. — Sphenoid  Bone  from  Embryo  of  3H  to  4  Months. 
The  parts  which  are  still  cartilaginous  are  represented  in  black,     as,  Alisphe- 
noid;  b,  basisphenoid;  I,  lingula;  os,  orbitosphenoid;  p,  internal  pterygoid  plate. — 
(Sappey.) 

owes  its-  origin  to  the  fusion  of  two  or  sometimes  four  centers  of 
ossification,  appearing  in  the  membranous  roof  of  the  embryonic 
skull.  The  bone  so  formed  (ip)  represents  the  interparietal  of 
lower  vertebrates  and,  at  an  early  stage,  unites  with  the  supra- 
occipital,  although  even  at  birth  an  indication  of  the  line  of  union 
of  the  two  parts  is  to  be  seen  in  two  deep  incisions  at  the  sides  of 
the  bone.  The  union  of  the  exoccipitals  and  supraoccipital  takes 
place  in  the  course  of  the  first  or  second  year  after  birth,  but  the 
basioccipital  does  not  fuse  with  the  rest  of  the  bone  until  the  sixth 
or  eighth  year.  It  will  be  noticed  that  no  special  centers  occur 
for  the  four  occipital  vertebrae,  these  structures  having  become 
completely  incorporated  in  the  chondrocranium,  and  even  the 
cartilaginous  partitions  which  divide  the  hypoglossal  foramina 
usually  disappear  during  the  process  of  ossification. 


176  OSSIFICATION    OF   THE    CHONDROCRANIUM 

Two  pairs  of  centers  have  been  described  for  the  interparietal 
bone  and  it  has  been  claimed  that  the  deep  lateral  incisions  divide 
the  lower  pair,  so  that  when  the  incisions  meet  and  persist  as  the 
sutura  mendosa,  separating  the  so-called  inca  bone  from  the  rest  of 
the  ^occipital,  the  division  does  not  correspond  to  the  line  between 
the  supraoccipital  and  the  interparietal,  but  a  portion  of  the  latter 
bone  remains  in  connection  with  the  supraoccipital.  Mall,  how- 
ever, in  twenty  preparations,  found  but  a  single  pair  of  centers  for 
the  interparietal. 

Occasionally  an  additional  pair  of  small  centers  appear  for  the 
uppermost  angle  of  the  interparietal,  and  the  bones  formed  from 
them  may  remain  distinct  as  what  have  been  termed  fonianelle  bones. 

In  the  sphenoidal  region  the  number  of  distinct  bones  which 
develop  is  much  greater  than  in  the  occipital  region.  At  the  be- 
ginning of  the  second  month  a  center  appears  in  each  of  the  carti- 
lages which  represent  the  alisphenoids  (great  wings).  These 
cartilages  do  not,  however,  represent  the  entire  extent  of  the  great 
wings  and  their  ossification  gives  rise  only  to  those  portions  of 
the  bone  in  the  neighborhood  of  the  foramina  ovale  and  rotundum 
and  to  the  lateral  pterygoid  plates.  The  remaining  portions  of 
the  wings,  the  orbital  and  temporal  portions,  develop  as  mem- 
brane bone  (Fawcett)  and  early  unite  with  the  portions  formed 
from  the  cartilage.  At  the  end  of  the  second  month  a  center 
2ip^Q2iX^  in  edich.  orhito sphenoid  (lesser  wing)  cartilage  (Fig.  109,  os), 
and  a  little  later  a  pair  of  centers  {h),  placed  side  by  side,  are 
developed  in  the  cartilage  representing  the  posterior  portion  of  the 
body;  together  these  form  what  is  known  as  the  basisphenoid. 
Still  later  a  center  appears  on  either  side  of  the  basisphenoids  to 
form  the  lingulce  (/),  and  another  pair  appears  in  the  anterior  part 
of  the  cartilage,  between  the  orbitosphenoids,  and  represent  the 
presphenoid. 

In  addition  to  these  ten  centers  in  cartilage  and  the  membrane 
portion  of  the  alisphenoid,  two  other  membrane  bones  are  included 
in  the  adult  sphenoid.  Thus,  a  little  before  the  appearance  of  the 
center  for  the  alisphenoids  an  ossification  is  formed  in  the  mesen- 
chyme of  each  lateral  wall  of  the  posterior  part  of  the  nasal  cavity 
and  gives  rise  to  the  medial  lamina  of  the  pterygoid  process,  the 
mesenchyme  at  the  tip  of  the  ossification  condensing  to  form  a 


OSSIFICATION    OF    THE    CHONDROCRANIUM 


177 


cartilaginous  hook-like  structure  over  which  the  tendon  of  the 
tensor  veli  palatini  plays.  This  cartilage  later  ossifies  to  form  the 
pterygoid  hamulus,  the  medial  pterygoid  lamina  being  thus  a 
combination  of  membrane  and  cartilage  bone,  the  latter,  however, 
being  a  secondary  development  and  quite  independent  of  the 
chondrocranium. 

By  the  sixth  month  the  lingulae  have  fused  with  the  basi- 
sphenoid  and  the  orbitosphenoids  with  the  presphenoid,  and  a 
little  later  the  basisphenoid  and  presphenoid  unite.  The  ali- 
sphenoids  and  medial  pterygoid  lam- 
inae remain  separate,  however,  until 
after  birth,  fusing  with  the  remaining 
portions  of  the  adult  bone  during  the 
first  year. 

The  cartilage  of  the  ethmoidal 
region  of  the  chondrocranium  forms 
somewhat  later  than  the  other  por- 
tions and  consists  at  first  of  a  stout 
median  mass  projecting  downward 
and  forward  into  the  nasal  process 
(Fig.  no,  Ip),  and  two  lateral  masses 
{Im),  situated  one  on  either  side  in 
the  mesenchyme  on  the  outer  side  of 
each  olfactory  pit.  Ossification  of  the 
lateral  masses  or  ectethmoids  begins  relatively  early,  but  it  appears 
in  the  upper  part  of  the  median  cartilage  only  after  birth,  pro- 
ducing the  crista  galli  and  the  perpendicular  plate,  which  together 
form  what  is  termed  the  mesethmoid.  When  first  formed,  the  three 
cartilages  are  quite  separate  from  one  another,  the  olfactory  and 
nasal  nerves  passing  down  between  them  to  the  olfactory  pit,  but 
later  trabeculae  begin  to  extend  across  from  the  mesethmoid  to 
the  upper  part  of  the  ectethmoids  and  eventually  form  a  fenestrated 
horizontal  lamella  which  ossifies  to  form  the  cribriform  plate. 

The  lower  part  of  the  median  cartilage  does  not  ossify,  but  a 
center  appears  on  each  side  of  the  median  line  in  the  mesenchyme 
behind  and  below  its  posterior  or  lower  border.     From  these 


Pig.  no.  —  Anterior  Por- 
tion OF  THE  Base  of  the  Skull 
OF  A  6  TO  7  Months'  Embryo. 

The  shaded  parts  represent 
cartilage,  cp.  Cribriform  plate; 
/w,  lateral  mass  of  the  ethmoid; 
Ip,  perpendicular  plate;  of,  optic 
foramen;  os,  orbitosphenoid. — 
{After  von  Spee.) 


12 


178 


OSSIFICATION    OF    THE    CHONDROCRANIUM 


centers  two  vertical  bony  plates  develop  which  unite  by  their 
median  surfaces  below,  and  above  invest  the  lower  border  of  the 
cartilage  and  form  the  vomer.  The  portion  of  the  cartilage  which 
is  thus  invested  undergoes  resorption,  but  the  more  anterior 
portions  persist  to  form  the  cartilaginous  septum  of  the  nose. 
The  vomer,  consequently,  is  not  really  a  portion  of  the  chondro- 
cranium,  but  is  a  membrane  bone;  its  intimate  relations  with  the 
median  ethmoidal  cartilage,  however,  make  it  convenient  to 
consider  it  in  this  place. 

When  first  formed,  the  ectethmoids 
are  masses  of  spongy  bone  and  show  no 
indication  of  the  honeycombed  appear- 
ance which  they  present  in  the  adult 
skull.  This  condition  is  produced  by 
the  absorption  of  the  bone  of  each  mass 
by  evaginations  into  it  of  the  mucous 
membrane  lining  the  nasal  cavity.  This 
same  process  also  brings  about  the  for- 
mation of  the  curved  plates  of  bone 
which  project  from  the  inner  surfaces 
of  the  lateral  masses  and  are  known 
as  the  superior  and  middle  conchae 
(turbinated  bones).  The  inferior  and 
sphenoidal  conchae  are  developed  from  special  centers,  but 
belong  to  the  same  category  as  the  others,  being  formed  from 
portions  of  the  lateral  ethmoidal  cartilages  which  become  almost 
separated  at  an  early  stage  before  the  ossification  has  made  much 
progress.  Absorption  of  the  body  of  the  sphenoid  bone  to-  form 
the  sphenoidal  cells,  of  the  frontal  to  form  the  frontal  sinuses,  and 
of  the  maxillaries  to  form  the  maxillary  antra  is  also  produced  by 
outgrowths  of  the  nasal  mucous  membrane,  all  these  cavities,  as 
well  as  the  ethmoidal  cells,  being  continuous  with  the  nasal  cavities 
and  lined  with  an  epithelium  which  is  continuous  with  the  mucous 
membrane  of  the  nose. 

In  the  lower  mammalia  the  erosion  of  the  mesial  surface  of  the 
ectethmoidal  cartilages  results,  as  a  rule,  in  the  formation  of  five  con- 


PiG.  III. — The  Temporal 
Bone  at  Birth.  The  Styloid 
Process  and  Auditory  Ossi- 
cles are  not  Represented. 
p.  Petrous  bone;  s,  squamosal; 
/,  tympanic. — (Poirier.) 


OSSIFICATION    OF   THE    CHONDROCRANIUM  1 79 

chae,  while  in  man  but  three  are  usually  recognized.  Not  infrequently, 
however,  indications  of  an  additional  concha  above  what  is  ordinarily 
termed  the  superior  concha  is  observed  in  man,  and  in  front  of  the 
superior  concha  a  slight  elevation,  termed  the  agger  nasi,  is  always 
observable  and  its  lower  edge  is  prolonged  downwards  and  backwards 
to  form  what  is  termed  the  uncinate  process  of  the  ethmoid.  This  proc- 
ess and  the  agger  together  represent  another  concha  of  the  typical  mam- 
malian arrangement,  to  which,  therefore,  the  human  arrangement  may 
be  reduced. 


A  number  of  centers  of  ossification — the  exact  number  is  yet 
uncertain — appear  in  the  periotic  capsule  during  the  later  portions 
of  the  fifth  month,  and  during  the  sixth  month  these  unite  together 
to  form  a  single  center  from  which  the  complete  ossification  of  the 
cartilage  proceeds  to  form  the  petrous  and  mastoid  portions  of  the 
temporal  bone  (Fig.  in,  ^).  The  mastoid  process  does  not  really 
form  until  several  years  after  birth,  being  produced  by  the  hollow- 
ing and  bulging  out  of  a  portion  of  the  petrous  bone  by  outgrowths 
from  the  lining  membrane  of  the  middle  ear.  The  cavities  so 
formed  are  the  mastoid  cells,  and  their  relations  to  the  middle-ear 
cavity  are  in  all  respects  similar  to  those  of  the  ethmoidal  and 
sphenoidal  cells  to  the  nasal  cavities.  The  remaining  portions 
of  the  temporal  bone  are  partly  formed  by  membrane  bone  and 
partly  from  the  branchial  arch  skeleton.  An  ossification  appears 
at  the  close  of  the  eighth  week  in  the  membrane  which  forms  the 
side  of  the  skull  in  the  temporal  region  and  gives  rise  to  a  squa- 
mosal bone  {s),  which  later  unites  with  the  petrous  to  form  the 
squamosal  portion  of  the  adult  temporal,  and  another  membrane 
bone,  the  tympanic  (t),  develops  from  a  center  appearing  in  the 
mesenchyme  surrounding  the  external  auditory  meatus,  and  later 
also  fuses  with  the  petrous  to  form  the  floor  and  sides  of  the 
external  meatus,  giving  attachment  at  its  inner  edge  to  the 
tympanic  membrane.  Finally,  the  styloid  process  is  developed 
from  t];ie  upper  part  of  the  second  branchial  arch,  whose  history 
will  be  considered  later. 

The  various  ossifications  which  form  in  the  chondrocranium 
and  the  portions  of  the  adult  skull  which  represent  them  are 
shown  in  the  following  table. 


i8o 


THE  MEMBRANE  BONES  OF  THE  SKULL 


Region  of 

Chondrocranium 

Ossification 

Parts  of  Adult  Skull 

Basioccipital 

Basilar  process. 

Occipital 

Exoccipitals 

Condyles. 

Supraoccipital 

Squamous  portion  below  superior 
nuchal  line. 

Basisphenoid 

Presphenoid 

Body. 

Sphenoidal 

Lingulae 

Alisphenoids 

Greater  wings  (in  part). 

Orbitosphenoids 

Lesser  wings. 

Lamina  perpendicularis. 

Mesethmoid 

Crista  galli. 
Nasal  septum. 
Lateral  masses. 
Superior  concha. 

Ethmoidal 

Ectethmoids 

Middle  concha. 

Inferior  concha. 

Sphenoidal  concha. 

Periotic  capsule 

Mastoid. 
Petrous. 

The  Membrane  Bones  of  the  Skull. — In  the  membrane  form- 
ing the  sides  and  roof  of  the  skull  in  the  second  stage  of  its  develop- 
ment ossifications  appear,  which  give  rise,  in  addition  to  the  in- 
terparietal and  squamosal  bones  already  mentioned  in  connection 
with  the  occipital  and  temporal,  to  the  parietals  and  frontal. 
Each  of  the  former  bones  develops  from  a  single  center  which 
appears  at  the  end  of  the  eighth  week,  while  the  frontal  is  formed 
at  about  the  same  time  from  two  centers  situated  symmetrically 
on  each  side  of  the  median  line  and  eventually  fusing  completely 
to  form  a  single  bone,  although  more  or  less  distinct  indications 
of  a  median  suture,  the  metopic,  are  not  infrequently  present. 

Furthermore,  ossifications  appear  in  the  mesenchyme  of  the 
facial  region  to  form  the  nasal,  lachrymal,  and  zygomatic  bones, 
all  of  which  arise  from  single  centers  of  ossification.  In  the  case 
of  each  zygomatic  bone,  however,  three  osseous  thickenings  appear 
on  the  inner  surface  of  the  original  ossification,  which  then  dis- 
appears and  the  thickenings  unite  to  form  the  adult  bone,  though 
occasionally  one  or  more  of  their  lines  of  union  may  persist,  pro- 
ducing a  bipartite  or  tripartite  zygomatic. 


OSSIFICATION    OF   BRANCHIAL    ARCH    SKELETON  l8l 

The  vomer,  which  has  already  been  described,  belongs  also 
strictly  to  the  category  of  membrane  bones,  as  do  also  the  maxillae 
and  the  palatines;  these  latter,  however,  primarily  belonging  to^ 
the  branchial  arch  skeleton,  with  which  they  will  be  considered. 

The  purely  membrane  bones  in   the  skull,   are,   then,   the 
following: 

Interparietals Part  of  squamous  portion  of  occipital 

Pterygoids Medial  pterygoid  plates. 

Squamosals Squamous  portions  of  temporals. 

Tympanies Tympanic  plates  of  temporals. 

Parietals. 

Frontal. 

Nasals. 

Lachrymals. 

Zygomatics. 

Vomer. 

The  Ossification  of  the  Branchial  Arch  Skeleton. — It  has 

been  seen  (p.  173)  that  a  cartilaginous  bar  develops  only  in  the 
mandibular  process  of  the  first  branchial  arch.  In  the  maxillary 
process  no  cartilaginous  skeleton 
forms,  but  two  membrane  bones, 
the  palatine  and  maxilla,  are  devel- 
oped in  it,  their  cartilaginous  repre- 
sentatives, which  are  to  be  found  in 
lower  vertebrates,  having  been  sup- 

,     ,  -  .  r      1  Pig,     112. — Diagram    of   the 

pressed    by   a    condensation  of  the    ossifications    of    which    the 

development.      The      palatine      bone  Maxilla    is  Composed,   as  seen 

^  ^         ^  ^  FROM  THE  Outer  Surface.     The 

develops  from  a  single  center  of  OSsi-  Arrow  Passes  through  the  In- 

r      .'         u    i-  r  1-  '17  1  fraorbital     Canal, — (From    von 

fication,  but  for  each  maxilla  no  less    spee,  after  Sappey.) 
than  five  centers  have  been  described  * 

(Fig.  112).  One  of  these  gives  rise  to  so  much  of  the  alveolar  border 
of  the  bone  as  contains  the  bicuspid  and  molar  teeth;  a  second  forms 
the  nasal  process  and  the  part  of  the  alveolar  border  which  con- 
tains the  canine  tooth;  a  third  the  portion  which  contains  the 
incisor  teeth;  while  the  fourth  and  fifth  centers  lie  above  the  first 
and  give  rise  to  the  inner  and  outer  portions  of  the  orbital  plate 
and  the  body  of  the  bone.     The  first,  second,  fourth,  and  fifth 


182  OSSIFICATION    OF    BRANCHIAL   ARCH    SKELETON 

portions  early  unite  together,  but  the  third  center,  which  really 
lies  in  the  ventral  part  of  the  nasal  process,  remains  separate  for 
some  time,  forming  what  is  termed  the  premaxilla,  a  bone  which 
remains  permanently  distinct  in  the  majority  of  the  lower 
mammals. 

The  above  is  the  generally  accepted  view  as  to  the  development  of 
the  maxilla.  Mall,  however,  maintains  that  it  has  but  two  centers  of 
ossification,  one  giving  rise  to  the  premaxilla  and  the  other  to  the  rest 
of  the  bone.  The  maxillary  center  makes  its  appearance  about  the 
middle  of  the  sixth  week. 


ZrAT 


Fig.  113. — Model  of  Right  Half  of  Mandible  of  a  Fetus  95  mm.  in  Length 

SEEN    from    the    MeSIAL    SURFACE. 

Ci  and  C2,  Accessory  cartilages;  Ch.  T.,  chorda  tympani;  Cr.,  cartilage  for  coro- 
noid  process;  Cy.,  cartilage  for  condyloid  process;  Mai.,  malleus;  M.C.,  Meckel's 
cartilage;  N.Al.,  inferior  alveolar  nerve;  N.  Aur.,  auriculo-temporal  nerve;  N.L,. 
lingual  nerve;  N.  Mh,  mylo-hyoid  nerve;  N.T.,  trigeminal  nerve;  Sy.,  symphy- 
sis.—  (Low.) 

Since  the  condition  known  as  hare-lip  results  from  a  failure  of  the 
maxillary  process  to  unite  completely  with  the  frontonasal  process  (see 
p.  102),  and  since  the  premaxilla  develops  in  the  latter  and  the  maxilla 
in  the  former,  the  cleft  may  pass  between  these  two  bones  and  prevent 
their  union  (see  also  p.  286). 

The  upper  end  of  Meckel's  cartilage  passes  between  the  tym- 
panic bone  and  the  outer  surface  of  the  periotic  capsule  and  thus 
comes  to  lie  apparently  within  the  tympanic  cavity  of  the  ear; 
this  portion  of  the  cartilage  divides  into  two  parts  which  ossify 
to  form  two  of  the  bones  of  the  middle  ear,  the  malleus  and  incus, 
a  description  of  whose  further  development  may  be  postponed 


OSSIFICATION    OF   BRANCHIAL    ARCH    SKELETON 


183 


until  a  later  chapter.  At  about  the  middle  of  the  sixth  week  of 
development  a  plate  of  membrane  bone  appears  to  the  outer  side 
of  the  lower  portion  of  the  cartilage  and  gradually  extends  to 
form  the  body  and  ramus  of  the  mandible. 


Fig.  114. — Diagram  showing  the  Categories  to  which  the  Bones  of  the  Skull 

Belong. 

The  unshaded  bones  .are  membrane  bones,  the  heavily  shaded  represent  the 
chondrocranium,  while  the  black  represents  the  branchial  arch  elements.  AS,  Ali- 
phenoid;  ExO,  exoccipital;  F,  frontal;  Hy,  hyoid;  IP,  interparietal;  Z,  zygomatic; 
Mn,  mandible;  Mx,  maxilla;  NA,  nasal;  P,  parietal;  Pe,  periotic;  SO,  supraoccipital; 
Sq,  squamosal;  St,  styloid  process;  Th,  thyreoid  cartilage;  Ty,  tympanic. 

In  the  region  of  the  body  the  bone  develops  so  as  to  enclose  the 
cartilage,  together  with  the  inferior  alveolar  (dental)  nerve  which 
lies  to  the  outer  side  of  the  cartilage,  but  in  the  region  of  the 
ramus  the  bone  remains  entirely  to  the  outer  side  of  the  cartilage 


1 84  OSSIFICATION    OF  BRANCHIAL   ARCH   SKELETON 

and  nerve,  whence  the  position  of  the  mandibular  foramen  on  the 
inner  surface  of  the  adult  bone.  The  anterior  portion  of  Meckel's 
cartilage  becomes  ossified  by  the  extension  of  ossification  from  the 
membrane  bone  into  it,  the  portion  corresponding  to  the  body 
of  the  bone  behind  the  mental  foramen  disappears  and  the  portion 
above  the  mandibular  foramen  is  said  to  become  transformed  into 
fibrous  connective  tissue  and  to  persist  as  the  spheno-mandibular 
ligament.  At  the  upper  extremity  of  the  ramus  two  nodules  of 
cartilage  develop,  quite  independently,  however,  of  Meckel's 
cartilage  (Fig.  113,  Cr  and  Cy),  and  ossification  extends  into  these 
from  the  ramus  to  form  the  coronoid  and  condyloid  processes. 
And,  finally,  two  other  independent  cartilages  appear  toward  the 
anterior  extremity  of  each  half  of  the  bone,  one  at  the  alveolar 
(Ci)  and  the  other  at  the  lower  border  (C2),  and  these  also  are 
later  incorporated  into  the  bone  without  developing  special  cen- 
ters of  ossification. 

Each  half  of  the  mandible  thus  ossifies  from  a  single  center,  and 
is  essentially  a  membrane  bone  replacing  a  cartilaginous  precursor. 
At  birth  the  two  halves  are  united  at  the  symphysis  by  fibrous 
tissue,  into  which  ossification  extends  later,  union  occurring  in  the 
first  or  second  year. 

The  upper  part  of  the  cartilage  of  the  second  branchial  arch 
also  comes  into  relation  with  the  tympanic  cavity  and  ossifies  to 
form  the  styloid  process  of  the  temporal  bone.  The  succeeding 
moiety  of  the  cartilage  undergoes  degeneration  to  form  the  stylo- 
hyoid ligament,  while  its  most  ventral  portion  ossifies  as  the  lesser 
cornu  of  the  hyoid  bone.  The  great  variability  which  may  be 
observed  in  the  length  of  the  styloid  processes  and  of  the  lesser  cor- 
nua  of  the  hyoid  depends  upon  the  extent  to  which  the  ossification 
of  the  original  cartilage  proceeds,  the  length  of  the  stylo-hyoid 
ligaments  being  in  inverse  ratio  to  the  length  of  the  processes  or 
cornua.  The  greater  cornua  of  the  hyoid  are  formed  by  the  ossifi- 
cation of  the  cartilages  of  the  third  arch,  and  the  body  of  the  bone 
is  formed  from  a  cartilaginous  plate,  the  copula,  which  unites  the 
ventral  ends  of  the  two  arches  concerned. 

Finally,  the  cartilages  of  the  fourth  and  fifth  branchial  arches 


isf  arch. 


DEVELOPMENT    OF    THE    APPENDICULAR    SKELETON  1 85 

early  fuse  together  to  form  a  plate  of  cartilage,  and  the  two  plates 
of  opposite  sides  unite  by  their  ventral  edges  to  form  the  thyreoid 
cartilage  of  the  larynx. 

The  accompanying  diagram  (Fig.  114)  shows  the  various 
structures  derived  from  the  branchial  arch  skeleton,  as  well  as 
some  of  the  other  elements  of  the  skull,  and  a  resume  of  the  fate  of 
the  branchial  arches  may  be  stated  in  tabular  form  as  follows,  the 
parts  represented  by  cartilage  which  becomes  replaced  by  mem- 
brane bone  being  printed  in  itahcs,  while  membrane  bones  which 
have  no  cartilaginous  representatives  are  enclosed  in  brackets : 

(Maxilla). 

(Palatine). 

Malleus. 

Incus. 

Spheno-mandibular  ligament. 

Mandible. 

I  Styloid  process  of  the  temporal. 
Stylo-hyoid  ligament. 
Lesser  cornu  of  hyoid. 

3d  arch Greater  cornu  of  hyoid. 

4th  and  5th  arches Thyreoid  cartilage  o^  larynx. 

The  Development  of  the  Appendicular  Skeleton. — While  the 
greater  portion  of  the  axial  skeleton  is  formed  from  the  sclerotomes 
of  the  mesodermic  somites,  the  appendicular  skeleton  is  derived 
from  the  somatic  mesenchyme,  which  is  not  divided  into  meta- 
meres.  This  mesenchyme  forms  the  core  of  the  limb  bud  and 
becomes  converted  into  cartilage,  by  the  ossification  of  which  all 
the  bones  of  the  limbs,  with  the  possible  exception  of  the  clavicle, 
are  formed. 

The  clavicle  is  the  first  bone  of  the  skeleton  to  ossify,  two  centers 
appearing  for  each  bone  at  about  the  sixth  week  of  development. 
Before  ossification  the  bone  is  represented  by  a  bar  of  tissue 
of  pecuKar  character,  it  being  difficult  to  say  whether  it  is  to  be 
regarded  as  cartilage  that  has  not  become  thoroughly  differentiated 
histologically,  or  as  some  special  variety  of  connective  tissue.  In 
this  the  two  centers  of  ossification  appear,  one  corresponding  to 


i86 


DEVELOPMENT  OF  THE  APPENDICULAR  SKELETON 


the  sternal  and  the  other  to  the  acromial  end  of  the  bone,  and  these 
are  at  first  united  by  a  bridge  of  the  original  tissue.  This  ossi- 
fies later,  so  that  the  two  centers  are  united,  and  at  either  end  of 
the  bone  true  cartilage  appears,  into  which  the  ossification  extends. 

This  mode  of  development  from  two  centers  explains  a  defect  occa- 
sionally observed  in  this  bone,  one  or  other  of  its  portions,  usually  the 
acromial  one,  being  wanting.  This  is  the  result  of  a  failure  of  the 
acromial  center. 


Fig.   115. — The  Ossification 

Centers  of  the  Scapula. 
a,  b,  and  c.  Secondary  centers  for 
the  angle,  vertebral  border,  and  acro- 
mion; CO,  center  for  the  coracoid  proc- 
ess.— (Testut.) 


Fig.  116. — Reconstruction  of  an 

Embryonic  Carpus. 
c,  Centrale;  cu,  triquetral;  lu,  lunate 
m,  capitate;  p,  pisiform;  sc,  navicular;  t, 
greater   multangular;    tr,   lesser   multan- 
gular; u,  hamate. 


The  scapula  is  at  first  a  single  plate  of  cartilage  in  which  two 
centers  of  ossification  appear.  One  of  these  gives  rise  to  the  body 
and  the  spine,  while  the  other  produces  the  coracoid  process 
(Fig.  115,  co)y  the  rudimentary  representative  of  the  coracoid 
bone  which  extends  between  the  scapula  and  sternum  in  the  lower 
vertebrates.  The  coracoid  does  not  unite  with  the  body  until 
about  the  fifteenth  year,  and  secondary  centers  which  give  rise 
to  the  vertebral  edge  (b)  and  inferior  angle  of  the  bone  (a)  and  to 
the  acromion  process  (c)  unite  with  the  rest  of  the  bone  at  about 
the  twentieth  year. 


DEVELOPMENT  OF  THE  APPENDICULAR  SKELETON     1 87 

The  humerus  and  the  bones  of  the  forearm  are  typical  long 
bones,  each  of  which  develops  from  a  primary  center,  which  gives 
rise  to  the  shaft,  and  has,  in  addition,  two  or  more  epiphysial 
centers.  In  the  humerus  an  epiphysial  center  appears  for  the 
head,  another  for  the  greater  tuberosity,  and  usually  a  third  for 
the  lesser  tuberosity,  while  at  the  distal  end  there  is  a  center  for 
each  epicondyle,  one  for  the  trochlea  and  one  for  the  capitulum,  the 
fusion  of  these  various  epiphyses  with  the  shaft  taking  place 
between  the  seventeenth  and  twentieth  years.  The  radius  and 
ulna  each  possesses  a  single  epiphysial  center  for  each  extremity  in 
addition  to  the  primary  center  for  the  shaft,  the  proximal  epi- 
physial center  for  the  ulna  giving  rise  to  the  tip  of  the  olecranon 
process. 

The  embryological  development  of  the  carpus  is  somewhat 
complicated.  A  cartilage  is  found  representing  each  of  the  bones 
normally  ocurring  in  the  adult  (Fig.  ii6),  and  these  are  arranged 
in  two  distinct  rows :  a  proximal  one  consisting  of  three  elements, 
named  from  their  relation  to  the  bones  of  the  forearm,  radiale^ 
intermedium,  and  ulnare;  and  a  distal  one  composed  of  four 
elements,  termed  carpalia.  In  addition,  a  cartilage,  termed  the 
pisiform,  is  found  on  the  ulnar  side  of  the  proximal  row  and  is 
generally  regarded  as  a  sesamoid  cartilage  developed  in  the  tendon 
of  the  flexor  carpi  ulnaris,  and  furthermore  a  number  of  inconstant 
cartilages  have  been  observed  whose  significance  in  the  majority 
of  cases  is  more  or  less  obscure.  These  accessory  cartilages  either 
disappear  in  later  stages  of  development  or  fuse  with  neighboring 
cartilages,  or,  in  rare  cases,  ossify  and  form  distinct  elements  of 
the  carpus.  One  of  them,  however,  occurs  so  frequently  as  almost 
to  deserve  classification  as  a  constant  element;  it  is  known  as 
the  cent  rale  (Fig.  ii6,  c)  and  occupies  a  position  between  the  carti- 
lages of  the  proximal  and  distal  rows  and  apparently  corresponds 
to  a  cartilage  typically  present  in  lower  forms  and  ossifying  to 
form  a  distinct  bone.  In  the  human  carpus  its  fate  varies,  as 
it  may  either  disappear  or  unite  with  other  cartilages,  that  with 
which  it  most  usually  fuses  being  probably  the  radiale.  There  is 
evidence  also  to  show  that  another  of  the  accessory  cartilages  unites 


DEVELOPMENT  OF  THE  APPENDICULAR  SKELETON 


with  the  ulnar  element  of  the  distal  row,  representing  the  carpale 
V  typically  present  in  lower  forms. 

Each  of  the  elements  corresponding  to  an  adult  bone  ossifies 
from  a  single  center  with  the  exception  of  carpale  iv-v  which  has 
two  centers,  a  further  indication  of  its  composite  character.  The 
relation  of  the  cartilages  to  the  adult  bones  may  be  seen  from  the 
table  given  on  p.  190. 

With  regard  to  the  metacarpals 
and  phalanges,  it  need  merely  be 
stated  that  each  develops  from  a 
single  primary  center  for  the  shaft 
and  one  secondary  epiphysial 
center.  The  primary  center  ap- 
pears at  about  the  middle  of  the 
shaft  except  in  the  terminal 
phalanges,  in  which  it  appears  at 
the  distal  end  of  the  cartilage. 
The  epiphyses  for  the  metacar- 
pals are  at  the  distal  ends  of  the 
bones,  except  in  the  case  of  the 
metacarpal  of  the  thumb,  which 
resembles  the  phalanges  in  having 
its  epiphysis  at  the  proximal  end. 

Each  innominate  bone  appears 
as  a  somewhat  oval  plate  of 
cartilage  whose  long  axis  is  di- 
rected almost  at  right  angles  to  the  vertebral  column  and  which 
is  in  close  relation  with  the  fourth  and  fifth  sacral  vertebrae. 
As  development  proceeds  a  rotation  of  the  cartilage,  accom- 
panied by  a  slight  shifting  of  position,  occurs,  so  that  eventually 
the  plate  has  its  long  axis  almost  parallel  with  the  vertebral 
column  and  is  in  relation  with  the  first  three  sacrals.  Ossi- 
fication appears  at  three  points  in  each  cartilage,  one  in  the 
upper  part  to  form  the  ilium  (Fig.  iiy,il),  and  two  in  the  lower  part, 
the  anterior  of  these  giving  rise  to  the  pubis  (p),  while  the  pos- 
terior produces  the  ischium  (is).     At  birth  these  three  bones  are 


Fig.  117. — The  Ossification  Cen- 
ters OF  THE  Os  InNOMINATUM. 
a,  b,  c,  and  d.  Secondary  centers 
for  the  crest,  anterior  inferior  spine, 
symphysis,  and  ischial  tuberosity;  il, 
ilium;  is,  ischium;  p,  pubis. — (Testut.) 


DEVELOPMENT  OF  THE  APPENDICULAR  SKELETON     1 89 

still  separated  from  one  another  by  a  Y-shaped  piece  of  cartilage 
whose  three  limbs  meet  at  the  bottom  of  the  acetabulum,  but 
later  a  secondary  center  appears  in  this  cartilage  and  unites  the 
three  bones  together.  The  central  part  of  the  lower  half  of  each 
original  cartilage  plate  does  not  undergo  complete  chondrifica- 
tion,  but  remains  membranous,  constituting  the  obturator  mem- 
brane which  closes  the  obturator  foramen. 

In  addition  to  the  Y-shaped  secondary  center,  other  epiphysial 
centers  appear  in  the  prominent  portions  of  the  cartilage,  such  as 
the  pubic  crest  (Fig.  117,  c,)  the  ischial  tuberosity  (d),  the  anterior 
inferior  spine  {b)  and  the  crest  of  the  iHum  (a),  and  unite  with  the 
rest  of  the  bone  at  about  the  twentieth  year. 

The  femur,  tibia,  smd  fibula  each  develop  from  a  single  primary 
center  for  the  shaft  and  an  upper  and  a  lower  epiphysial  center,  the 
femur  possessing,  in  addition,  epiphysial  centers  for  the  greater 
and  lesser  trochanters  (Fig.  94).  The  patella  does  not  belong  to 
the  same  category  as  the  other  bones,  but  resembles  the  pisiform 
bone  of  the  carpus  in  being  a  sesamoid  bone,  developed  in  the 
tendon  of  the  quadriceps  extensor  cruris.  Its  cartilage  does  not 
appear  until  the  fourth  month  of  intrauterine  life,  when  most  of 
the  primary  centers  for  other  bones  have  already  appeared,  and 
its  ossification  does  not  begin  until  the  third  year  after  birth. 

The  tarsus,  like  the  carpus,  consists  of  a  proximal  row  of  three 
cartilages,  termed  the  tibiale,  the  intermedium,  and  the fibulare,  and 
of  a  distal  row  of  four  tarsalia.  Between  these  two  rows  a  single 
cartilage,  the  centrale,  is  interposed.  Each  of  these  cartilages 
ossifies  from  a  single  center,  that  of  the  intermedium  early  fusing 
with  the  tibiale,  though  it  occasionally  remains  distinct  as  the  os 
trigonum,  and  from  a  comparison  with  lower  forms  it  seems 
probable  that  the  fibular  cartilage  of  the  distal  row  really  repre- 
sents two  separate  elements,  there  being,  properly  speaking,  five 
tarsaHa  instead  of  four.  The  fibulare,  in  addition  to  its  primary 
center,  possesses  also  an  epiphysial  center,  which  develops  at  the 
point  of  insertion  of  the  tendo  AchiUis. 

A  comparison  of  the  carpal  and  tarsal  cartilages  and  their 
relations  to  the  adult  bones  may  be  seen  from  the  following  table ; 


IQO 


DEVELOPMENT    OF   THE   JOINTS 


Carpus 

Tarsus 

Cartilages 

Bones 

Bones 

Cartilages 

Radiale 

Navicular 

Talus 

1  Tibiale 

I  Intermedium 

Intermedium 

Lunate 

Ulnare 

Triquetral 

Calcaneus 

Fibulare 

Sesamoid  cartilage 

Pisiform 

Centrale 

Fuses    with    navi- 
cular 

Navicular 

Centrale 

Carpale  I 

Gr,  multangular 

ist  Cuneiform 

Tarsale  I 

Carpale  II 

Less,  multangular 

2d  Cuneiform 

Tarsale  II 

Carpale  III 

Capitate 

3d  Cuneiform 

Tarsale  III 

Carpale  IV 
CarpaleV    , 

Hamate 

Cuboid 

f  Tarsale  IV 
1  Tarsale  V 

The  development  of  the  metatarsals  and  phalanges  is  exactly 
similar  to  that  of  the  corresponding  bones  of  the  hand  (see  p.  182) . 

The  Development  of  the  Joints. — The  mesenchyme  which 
primarily  represents  each  vertebra,  or  the  skull,  or  the  skeleton  of 
a  limb,  is  at  first  a  continuous  mass,  and  when  it  becomes  con- 
verted into  cartilage  this  also  may  be  continuous,  as  in  the  skull, 
or  may  appear  as  a  number  of  distinct  parts  united  by  unmodified 
portions  of  the  mesenchyme.  In  the  former  case  the  various  ossifi- 
cations as  they  extend  will  come  into  contact  with  their  neigh- 
bors and  will  either  fuse  with  them  or  will  articulate  with  them 
directly,  forming  a  suture. 

When,  however,  a  portion  of  unmodified  mesenchyme  inter- 
venes between  two  cartilages,  the  mode  of  articulation  of  the 
bones  formed  from  these  cartilages  will  vary.  The  intermediate 
mesenchyme  may  in  time  undergo  chondrification  and  unite  the 
bones  in  an  almost  immovable  articulation  known  as  a  syn- 
chondrosis {e.g.,  the  articulation  of  the  first  rib  with  the  sternum); 
or  a  cavity  may  appear  in  the  center  of  the  intervening  cartilage 
so  that  a  slight  amount  of  movement  of  the  two  bones  is  possible, 
forming  an  amp  hiar  thro  sis  (e.g.,  the  symphysis  pubis);  or,  finally, 
the  intermediate  mesenchyme  may  not  chondrify,  but  its  per- 
ipheral portions  may  become  converted  into  a  dense  sheath  of 
connective  tissue  (Fig.  118,  c)  which  surrounds  the  adjacent  ends 


DEVELOPMENT    OF    THE   JOINTS  IQI 

of  the  two  bones  like  a  sleeve,  forming  the  articular  capsule,  while 
the  central  portions  degenerate  to  form  a  cavity.  The  bones 
which  enter  into  such  an  articulation  are  more  or  less  freely  mov- 
able upon  one  another  and  the  joint  is  termed  a  diarthrosis  {e.g., 
the  knee-  or  shoulder-joint). 

In  a  diarthrosis  the  connective-tissue  cells  near  the  inner 
surface  of  the  capsule  arrange  themselves  in  a  layer  to  form  a 
synovial  membrane  for  the  joint,  and  portions  of  the  capsule  may 
thicken  to  form  special  bands,  the  reinforcing  ligaments,  while 
other  strong  fibrous  bands,  which  may  pass  from  one  bone  to  the 
other,  forming  accessory  Ugaments,  are  shown  by  comparative 


c~~\ 


Fig.  1 1 8. — Longitudinal  Section  through  the  Joint  of  the  Great  Toe  in  an 

Embryo  of  4.5  cm. 

c.  Articular  capsule;  i,  intermediate  mesenchyme  which  has  almost  disappeared  in 

the  center;  p^  and  p^,  cartilages  of  the  first  and  second  phalanges. — (Nicholas.) 

studies  to  be  in  many  cases  degenerated  portions  of  what  were 
originally  muscles. 

In  certain  diarthroses,  such  as  the  temporo-mandibular  and 
sterno-clavicular,  the  whole  of  the  central  portions  of  the  inter- 
mediate mesenchyme  does  not  degenerate,  but  it  is  converted  into  a 
fibro-cartilage,  between  each  surface  of  which  and  the  adjacent 
bone  there  is  a  cavity.  These  interarticular  cartilages  seem,  in 
the  sterno-clavicular  joints,  to  represent  the  sternal  ends  of  a  bone 
existing  in  lower  vertebrates  and  known  as  the  precoracoid,  but 
it  seems  doubtful  if  those  of  the  temporo-mandibular  and  knee- 
joints  have  a  similar  significance,  the  most  recent  observations  on 


192  LITERATURE 

their  development  tending  to  derive  them  from  the  intermediate 
mesenchyme. 

From  their  mode  of  development  it  is  evident  that  the  cavities  of 
diarthrodial  joints  are  completely  closed  and  their  walls,  except  where 
they  are  formed  by  cartilage,  are  lined  by  a  continuous  layer  of  synovial 
cells.  Ligaments  or  tendons,  which,  at  first  sight,  appear  to  traverse 
the  cavities  of  certain  joints,  are  in  reality  excluded  from  them,  being 
lined  by  a  sheath  of  synovial  cells  continuous  with  the  layer  lining  the 
general  cavity.  Thus,  the  tendon  of  the  long  head  of  the  biceps,  which 
seems  to  traverse  the  shoulder-joint  is,  in  the  fetus,  entirely  outside 
the  articular  capsule,  upon  which  it  tests.  Later  it  sinks  in  toward  the 
joint  cavity,  pushing  the  articular  capsule  before  it,  so  that  it  lies  at 
first  in  a  groove  in  the  capsule,  which  later  on  becomes  converted  into 
a  canal  and,  finally,  separates  from  the  rest  of  the  capsule  except  at  its 
extremities,  forming  a  cylindrical  canal,  open  at  either  end,  traversing 
the  joint  cavity  and  containing  the  tendon  of  the  biceps. 

The  ligamentum  teres  of  the  hip-joint  is  similarly  excluded  from  the 
joint  cavity  by  a  sheath  of  synovium,*  which  extends  outward  around  it 
from  the  bottom  of  the  acetabular  fossa  to  the  depression  in  the  head 
of  the  femur,  and  in  the  knee-joint  the  crucial  ligaments  are  also  ex- 
cluded from  the  cavity  by  a  reflection  of  the  synovium.  This  joint, 
indeed,  is  in  the  fetus  incompletely  divided  into  two  parts,  one  corre- 
sponding to  each  femoral  condyle,  by  a  partition  which  extends  back- 
ward from  the  patellar  ligament  to  the  crucial  ligaments,  remains  of 
this  partition  persisting  in  the  adult  as  the  so-called  ligamentum 
mucosum. 

LITERATURE 

L.  B.  Arey:     The  Origin,  Growth,  and  Fate  of  Osteoclasts  and  Their  Relation  to 

Bone  Resorption,"  Amer.  Journ.  Anal.,  xxvi,  1920. 
C.  R.  Bardeen:  "The  Development  of  the  Thoracic  Vertebrae  in  Man,"  Amer. 

Journ.  Anat.,  iv,  1905. 
C.  R.  Bardeen:  "Studies  of  the  Development  of  the  Human  Skeleton,"  Amer. 

Journ.  Anat.,  rv,  1905. 
G.  R.  Bardeen:  "Early  Development  of  the  Cervical  Vertebrae  and  the  Base  of 

Occipital  Bone  in  Man,"  Amer.  Journ.  Anat.,  viii,  1908. 
C.  R.  Bardeen:  "Vertebral  Regional  Determination  in  Young  Human  Embryos," 

Anat.  Record,  11,  1908. 
E.  T.  Bell:  "On  the  Histogenesis  of  the  Adipose  Tissue  of  the  Ox,"  Amer.  Journ. 

Anat.,  IX,  1909. 
A.   Bernays:  "Die  Entwicklungsgeschichte  des  Kniegelenks  des  Menschen  mit 

Bemerkungen  iiber  die  Gelenke  im  Allgemeinen,"  Morpholog.  Jahrbuch,  iv,  1878. 
E.  Dursy:  "Zur  Entwicklungsgeschichte  des  Kopfes  des  Menschen  und  der  hoheren 

Wirbelthiere,"  Tubingen,  1869. 
E.  Fawcett:  "On  the  Development,  Ossification  and  Growth  of  the  Palate  Bone," 

Journ.  Anat.  and  Phys.,  xl,  1906, 


LITERATURE  I93 

E.  Fawcett:  "Notes  on  the  Development  of  the  Human  Sphenoid,"  Journ.  Anat. 

and  Phys.y  xliv,  1910. 
E.  Fawcett:  "The  Development  of  the  Human  Maxilla,  Vomer  and  Paraseptal  — 

Cartilages,"  Journ.  Anat.  andPhys.,XLV,  1911. 
E.  Fawcett:  "The  Development  and  Ossification  of  the  Human  Clavicle,"  Journ. 

Anat.  and  Phys.,  xlvii,  1913. 
A.  Froriep:  "Zur  Entwicklungsgeschichte  der  Wirbelsaule,  insbesondere  des  Atlas 

und  Epistropheus  und  der  Occipitalregion,"  ArchivfUr  Anat.  und  Physiol.,  Anat. 

Abth.,  1886. 
E.  Gaupp:  "Alte    Probleme    und  neuere  Arbeiten  uber  den  Wirbeltierschadel," 

Ergeb.  der  Anat.  und  Entwicklungsgesch.,  x,  1901. 
C.  Gegenbaur:  "Ein  Fall  von  erblichem  Mangel  der  Pars  acromialis  Claviculae,  mit 

Bemerkungen   iiber   die    Entwicklung   der  Clavicula,"  Jenaische  Zeitschr.  fur 

medic.  Wissensch.,  i,  1864. 
J.  GoLOWiNSKi;   "Zur  Kenntnis  der  Histogenese  der  Bindegewebsfibrillen,"  Anat. 

Hefte,  xxxni,  1907. 

E.  Grafenberg:  "Die  Entwicklung  der  Knochen,  Muskeln  und  Nerven  der  Hand 

und  der  fur  die  Bewegungen  der  Hand  bestimmten  Muskeln  des  Unterarms," 
Anat.  Hefte,  xxx,  1906. 

F.  B.  Hanson:  "The  Development  of  the  Sternum  in  Sus  Scrofa,    Anat.  Record, 

XVII,  1919. 
Henke   and   Reyher:  "Studien   uber   die   Entwickelung  der  Extremitaten   des 

Menschen,  insbesondere  der  Gelenkflachen,"  Sitzungsherickte  der  kais.     Akad. 

Wien,  Lxx,  1875. 
M.  Jakoby:  "Beitrage  zur  Kenntnis  des  menschlichen  Primordialcraniums,"  Archiv 

fur  mikrosk.  Anat.,  xliv,  1894. 
K.  Kjellberg:  "Beitrage  zur  Entwicklungsgeschichte  des  Kiefergelenks,"  Morph. 

Jahrbuch,  xxxii,  1904. 
H.  Leboucq:  "Recherches  sur  la  morphologic  du  carpe  chez  les  mammiferes," 

Archives  de  Biolog.,  v,  1884. 

G.  Levi:  "Beitrag  zum  Studium  der  Entwickelung  des  knorpeligen  Primordialcran- 

iums des  Menschen,"  Archiv  fur  mikrosk.  Anat.,  in,  1900. 
A.  Linck:  "Beitrage  zur  Kenntnis  der  menschlichen  Chorda  doralis  in  Hals-und 

Kopfskelett,  etc.,"  Anat.  Hefte,  xlii,  1911. 
A.  Low:  "Further  Observations  on  the  Ossification  of  the  Human  Lower  Jaw," 

Journ.  Anat.  and  Phys.,  xliv,  1910. 
C.  C.  Macklin:  "The  Skull  of  a  Human  Fetus  of  40  mm.,"  Amer.  Journ.  Anat.,  xvi, 

1914. 
M.  Lucien:  "  Developpement  de  I'articulation  du  genou  et  formation  du  ligament 

adipeux,"  Bibliogr.  Anat.,  xiii,  1904. 
F.  P.  Mall:  "The  Development  of  the  Connective  Tissues  from  the  Connective- 
tissue  Syncytium,"  Amer.  Jour.  Anat.,  i,  1902. 
F.  P.  Mall:  "On  Ossification  Centers  in  Human  Embryos  Less  Than  One  Hundred 

Days  Old,"  Amer.  Journ.  Anat.,  v,  1906. 
F.  Merkel:  "  Betrachtungen  iiber  die  Entwicklung  des  Bindegewebes,"  Anat.  Hefte, 

xxxviii,  1909. 

13 


194  LITERATURE 

^/W.  VAN  Noorden:  "Beitrag  zur  Anatomic  der  knorpeligen  Schadelbasis  menschlicher 

Embryonen,"  Archivfiir  Anat.  und Physiol.,  Anat.  Abih.,  1887. 
A.  M.  Paterson:  "The  Human  Sternum," Liverpool,  1904. 
K.  Peter:  "  Anlage  und  Homologie  der  Muscheln  des  Menschen  und  der  Saugetiere, 

Arch,  filr  mikrosk.  Anat.,  lx,  1902. 
J.  W.  Pryor:  "The  Chronology  and  Order  of  Ossification  of  the  Bones  of  the  Human 

Carpus,"  Bulletin  State  Univ.,  Lexington,  Ky.,  1908. 
Rambaut  et  Renault:  " Origine  et  developpement  des  Os,"  Paris,  1864. 
E.  Rosenberg:  "Ueber  die  Entwickelung  der  Wirbelsaule  und  das  Centrale  carpi 

des  Menschen,"  Morpholog.  Jahrbuch,  i,  1876. 
H.  AND  H.  Rouviere:  "Sur  le  developpement  de  I'antre  mastoidien  et  les  cellules 

mastoidiennes,"  Bihliogr.  Anat.,  xx,  1910. 
G.   Ruge:  "  Untersuchungen  iiber  die  Entwickelungsvorgange  am  Brustbein  des 

Menschen,"  Morpholog.  Jahrbuch,  vi,  1880. 
J.  P.  Schaffer:  "The  Lateral  Wall  of  the  Cavum  Nasi  in  Man,  with  Especial 

Reference  to  the  Various  Developmental  Stages,"  Journ.  Morph.,  xxi,  1910. 
J.  P.  Schaffer:  "The  Sinus  Maxillaris  and  its  Relations  in  the  Embryo,  Child  and 

Adult  Man,"  Amer.  Journ.  Anat.,  x,  1910. 
G.  Thilenius:  "Untersuchungen  iiber  die  morphologische  Bedeutung  accessorischer 

Elemente  am  menschlichen  Carpus  (und  Tarsus),"  Morpholog.  Arbeiten,  v,  1896. 
K.  ToLDT,  Jr.:  " Entwicklung  und  Struktur  des  menschlichen  Jochbeines,"  Sitz- 

ungsber.  k.  Acad.  Wissensch.  Wien,  Math.-naturwiss  KL,  cxi,  1902. 
A.     Vinogradoff:  "  D6veloppement     de    I'articulation     temporo-maxillaire     chez 

I'homme  dans  la  p^riode  intrauterine,"  Internal.  Monatsschr.  Anat.  Phys.,  xxvii, 

1910. 
R.  H.  Whitehead  and  J,  A.  Waddell:  "The  Early  Development  of  the  Mammalian 

Sternum,"  Amer.  Journ.  Anat.,  xii,  1911. 
L.  W.  Williams:  "The  Later  Development  of  the  Notochord,"  Amer.  Journ.  Anat., 

VIII,  1908. 
E.  Zuckerkandl:  "Ueber  den  Jacobsonschen  Knorpel  und  die  Ossifikation  des 

Pflugscharbeines,"  Sitzb.  Akad.  Wiss.  Wien.,  cxvii,  1908. 


CHAPTER  VIII 
THE  DEVELOPMENT  OF  THE  MUSCULAR  SYSTEM 

Two  forms  of  muscular  tissue  exist  in  the  human  body,  the 
striated  tissue,  which  forms  the  skeletal  muscles  and  is  under  the 
control  of  the  central  nervous  system,  and  the  non-striated,  which 
is  controlled  by  the  sympathetic  nervous  system  and  is  found  in 
the  skin,  in  the  walls  of  the  digestive  tract,  the  blood-vessels  and 
lymphatics,  and  in  connection  with  the  genito-urinary  apparatus. 
In  the  walls  of  the  heart  a  muscle  tissue  occurs  which  is  frequently 
regarded  as  a  third  form,  characterized  by  being  under  control  of 
the  sympathetic  system  and  yet  being  striated;  it  is,  however,  in 
its  origin  much  more  nearly  allied  to  the  non-striated  than  to  the 
striated  form  of  tissue,  and  will  be  considered  a  variety  of  the 
former. 

The  Histogenesis  of  Non-Striated  Muscular  Tissue.— ^With 
the  exception  of  the  sphincter  and  dilator  of  the  pupil  and  the 
muscles  of  the  sudoriparous  glands,  which  are  apparently  formed 
from  the  ectoderm,  all  the  non-striated  muscle  tissue  of  the  body  is 
formed  by  the  conversion  of  mesenchyme  cells  into  muscle-fibers. 
The  details  of  this  process  have  been  worked  out  by  McGill  for  the 
musculature  of  the  digestive  and  respiratory  tracts  of  the  pig 
and  are  as  follows:  The  mesenchyme  surrounding  the  mucosa  in 
these  tracts  is  at  first  a  loose  syncytium  (Fig.  119,  m)  and  in  the 
regions  where  the  muscle  tissue  is  to  form,  a  condensation  of  the 
mesenchyme  occurs  followed  by  an  elongation  of  the  mesenchyme 
cells  and  their  nuclei,  so  that  the  muscle  layers  become  clearly 
distinguishable  from  the  neighboring  undifferentiated  tissue  (Fig. 
119,  mm).  Fibrils  of  two  kinds  then  begin  to  appear  in  the  cyto- 
plasm of  the  muscle  cells.  Coarse  fibrils  (f.c)  make  their  appear- 
ance as  rows  of  granules,  which  enlarge  and  increase  in  number 
until  they  finally  fuse  to  form  homogeneous  fibrils  that  are  at 

195 


196        HISTOGENESIS    OF   NON-STRIATED   MUSCULAR   TISSUE 


Fig.  119. — Longitudinal  Section  of  the  Lower  Part  of  the  Oesophagus 
OF  a  Pig  Embryo  of  15  mm,  Showing  the  Histogenesis  of  the  Non-striated 
Musculature. 

b,  Basement  membrane;  e,  epithelium; /.c,  coarse  fibril;/,/.,  fine  fibril;  ga,  gang- 
lion of  Auerbach's  plexus;  gm,  ganglion  of  Meissner's  plexus;  m,  mesenchyme;  mm, 
muscularis  mucosae;  pb,  protoplasmic  bridge;  vf,  varicose  fibril. — (McGill.) 


HISTOGENESIS    OF   NON-STRIATED.  MUSCULAR    TISSUE 


197 


first  varicose,  but  later  become  of  uniform  caliber.  Fine  fibrils 
(/•/)  which  are  homogeneous  from  the  first,  make  their  appearance 
after  the  coarse  ones  and  in  some  cases  seem  to  be  formed  by  the 
splitting  of  the  latter.  They  are  scattered  uniformly  throughout 
the  cytoplasm  of  the  muscle  cells  and  increase  in  number  as 
development  proceeds,  while  the  coarse  fibrils  diminish  and  may 
be  entirely  wanting  in  the  adult  tissue. 

Some  of  the  mesenchyme 
cells  in  each  muscle  sheet  fail 
to  undergo  the  differentiation 
just  described  and  multiply  to 
form  the  interstitial  connective 
tissue,  which  usually  divides  the 
muscle  cells  into  more  or  less 
distinct  bundles.  Traces  of  the 
original  syncytial  nature  of  the 
tissue  are  to  be  seen  in  the 
intercellular  bridges  that  occur 
between  the  non-striated  muscle 
cells  of  many  adult  forms. 

The  cells  from  which  the 
heart  musculature  develops  are 
at  first  of  the  usual  well  defined 
embryonic  type,  but,  as  develop- 
ment proceeds,  they  become  ir- 
regularly stellate  in  form,  the 

processes  of  neighboring  cells  fuse  and,  eventually,  there  is  formed 
a  continuous  mass  of  protoplasm  or  syncytium  in  which  all  traces 
of  cell  boundaries  are  lacking  (Fig.  120).  While  the  individual  cells, 
or  myoblasts  as  they  are  termed,  are  still  recognizable,  granules 
appear  in  their  cytoplasm,  and  these  arrange  themselves  in  rows 
and  unite  to  form  slender  fibrils,  which  at  first  do  not  extend  be- 
yond the  limits  of  the  myoblasts  in  which  they  have  appeared,  but 
later,  as  the  fusion  of  the  cells  proceeds,  are  continued  from  one 
cell  territory  into  the  other  through  considerable  stretches  of  the 
syncytium,  without  regard  to  the  original  cell  areas. 


Fig.  120. — Section  through  the 
Heart-wall  of  a  Duck  Embryo  of 
Three  Days. — (M.  Heidenhain.) 


198 


HISTOGENESIS  jOF    STRIATED    MUSCLE    TISSUE 


The  fibrils  multiply,  apparently  by  longitudinal  division,  and 
arrange  themselves  in  circles  around  areas  of  the  syncytium  (com- 
pare Fig.  121).  As  the  multiplication  of  the  fibrils  continues  those 
newly  formed  arrange  themselves  around  the  interior  of  each  of 
the  original  circles  and  gradually  occupy  the  entire  cytoplasm,  or 
sarcoplasm  as  it  may  now  be  termed,  except  immediately  around 
the  nuclei  where,  even  in  the  adult,  a  certain  amount  of  un- 
differentiated sarcoplasm  persists.  The  fibrils  when  first  formed 
are  apparently  homogeneous,  but  later  they  become  differentiated 
into  two  distinct  substances  which  alternate  with  one  another 


Pig.  121. — Cross-section  OF  A  Muscle  FROM  THE  Thigh  OF  A  Pig  Embryo  75  mm. 

Long. 

A,  Central  nucleus;  B,  new  peripheral  nucleus. — (Macalhim.) 

throughout  the  length  of  the  fibril  and  produce  the  characteristic 
transverse  striation  of  the  tissue.  Finally  stronger  interrupted 
transverse  bands  of  so-called  cement  substance  appear,  dividing 
the  tissue  into  areas  which  have  usually  been  regarded  as  rep- 
resenting the  original  myoblasts,  but  are  really  devoid  of  signifi- 
cance as  cells,  the  tissue  remaining,  strictly  speaking,  a  syncytium. 
The  Histogenesis  of  Striated  Muscle  Tissue. — The  histo- 
genesis of  striated  or  voluntary  muscle  tissue  resembles  very 
closely  that  which  has  just  been  described  for  the  heart  muscle. 
There  is  a  similar  formation  of  a  syncytium  by  the  fusion  of  the 


DEVELOPMENT  OF  SKELETAL  MUSCLES  1 99 

cells  of  the  myotomes,  an  appearance  of  granules  which  unite  to 
form  fibrils,  an  increase  of  the  fibrils  by  longitudinal  division  and  a 
primary  arrangement  of  the  fibrils  around  the  periphery  of  areas 
of  sarcoplasm  (Fig.  121),  each  of  which  represents  a  muscle  fiber. 
In  addition  there  is  an  active  proliferation  of  the  nuclei  of  the 
original  myoblasts,  the  new  nuclei  arranging  themselves  more  or 
less  regularly  in  rows  and  later  migrating  from  their  original 
central  position  to  the  periphery  of  the  fibers,  and,  in  the  limb 
muscles,  the  development  is  further  complicated  by  a  process  of 
degeneration  which  affects  groups  of  muscle  fibers,  so  that  bundles 
of  normal  fibers  are  separated  by  strands  of  degenerated  tissue  in 
which  the  fibrils  have  disappeared,  the  nuclei  have  become  pale 
and  the  sarcoplasm  vacuolated  and  homogeneous.  Later  the 
degenerated  tissue  seems  to  disappear  entirely  and  mesenchyma- 
tous  connective  tissue  grows  in  between  the  persisting  fibers,  group- 
ing them  into  bundles  and  the  bundles  into  the  individual  muscles. 

So  long  as  the  formation  of  new  fibrils  continues,  the  increase 
in  the  thickness  of  a  muscle  is  probably  due  to  a  certain  extent 
to  an  increase  in  the  actual  number  of  fibers,  which  results  from 
the  division  of  those  already  existing.  Subsequently,  however, 
this  mode  of  growth  ceases,  the  further  increase  of  the  muscle 
depending  upon  an  increase  in  size  of  its  constituent  elements 
(Macallum). 

The  Development  of  the  Skeletal  Muscles. — It  has  already 
been  pointed  out  that  all  the  skeletal  muscles  of  the  trunk  are 
derived  from  the  myotomes  of  the  mesodermic  somites.  Those 
that  are  primarily  associated  with  the  branchial  arches,  however, 
have  their  origin  from  the  unsegmented  ventral  mesoderm  and 
this  seems  also  to  be  the  origin  of  the  muscles  developed  in  the 
limb  buds,  these  being  differentiated  from  the  somatic  mesen- 
chyme which  forms  the  axial  cores  of  the  limb  buds. 

The  various  fibers  of  each  myotome  are  at  first  loosely  arranged 
but  later  they  become  more  compact  and  are  arranged  parallel 
with  one  another,  their  long  axes  being  directed  antero-posteriorly. 
This  stage  is  also  transitory,  however,  the  fibers  of  each  myotome 
undergoing    various    modifications    to    produce    the    conditions 


200  DEVELOPMENT  OF  SKELETAL  MUSCLES 

existing  in  the  adult,  in  which  the  original  segmental  arrangement 
of  the  fibers  can  be  perceived  in  comparatively  few  muscles. 
The  exact  nature  of  these  modifications  is  almost  unknown  from 
direct  observation,  but  since  the  relation  between  a  nerve  and 
the  muscle  fibers  supplied  by  it  is  established  at  a  very  early 
period  of  development  and  persists  throughout  life  no  matter 
what  changes  of  fusion,  splitting,  or  migration  the  muscle  may 
undergo,  it  is  possible  to  trace  out  more  or  less  completely  the 
history  of  the  various  muscles  by  determining  their  segmental 
innervation.  It  is  known,  for  example,  that  the  latissimus  dorsi 
arises  in  the  region  of  the  seventh  and  eighth*  cervical  myotomes, 
but  later  undergoes  a  migration,  becoming  attached  to  the  lower 
thoracic  and  lumbar  vertebrae  and  to  the  crest  of  the  ilium,  far 
away  from  its  place  of  origin  (Mall),  and  yet  it  retains  its  nerve- 
supply  from  the  seventh  and  eighth  cervical  nerves  with  which 
it  was  originally  associated,  its  nerve-supply  consequently  indicat- 
ing the  extent  of  its  migration. 

By  following  the  indications  thus  afforded,  it  may  be  seen 
that  the  changes  that  tend  to  obscure  the  primary  segmental 
arrangement  of  the  muscle  fibers  may  be  referred  to  one  or  more 
of  the  following  processes: 

1.  A  longitudinal  splitting  into  two  or  more  portions,  a  process 
well  illustrated  by  the  trapezius  and  sternomastoid,  which  have 
differentiated  by  the  longitudinal  splitting  of  a  single  sheet  and 
contain  therefore  portions  of  the  same  muscle-segments.  The 
sternohyoid  and  omohyoid  have  also  differentiated  by  the  same 
process,  and,  indeed,  it  is  of  frequent  occurrence. 

2.  A  tangential  splitting  into  two  or  more  layers.  Examples 
of  this  are  also  abundant  and  are  afforded  by  the  muscles  of  the 
fourth,  fifth,  and  sixth  layers  of  the  back,  as  recognized  in  English 
text-books  of  anatomy,  by  the  two  oblique  and  the  transverse 
layers  of  the  abdominal  walls,  and  by  the  intercostal  muscles  and 
the  transversus  of  the  thorax. 

*  This  enumeration  is  based  on  convenience  in  associating  the  myotomes  with  the 
nerves  which  supply  them.  The  myotomes  mentioned  are  those  which  correspond  to 
the  sixth  and  seventh  cervical  vertebrae.  .    ,  < 


DEVELOPMENT  OF  SKELETAL  MUSCLES  20I 

3.  A  fusion  of  portions  of  successive  myotomes  to  form  a  single 
muscle,  again  a  process  of  frequent  occurrence,  and  well  illus- 
trated by  the  rectus  abdominis  (which  is  formed  by  the  fusion 
of  the  ventral  portions  of  the  last  six  or  seven  thoracic  myotomes), 
or  by  the  superficial  portions  of  the  sacro-spinalis. 

4.  A  migration  of  parts  of  one  or  more  muscle-segments  over 
others.  An  example  of  this  process  is  to  be  found  in  the  latissimus 
dorsi,  whose  history  has  already  been  referred  to,  and  it  is  also 
beautifully  shown  by  the  serratus  anterior  and  the  trapezius, 
both  of  which  have  extended  far  beyond  the  limits  of  the  segments 
from  which  they  are  derived. 

5.  A  degeneration  of  portions  or  the  whole  of  a  muscle-segment. 
This  process  has  played  a  very  considerable  part  in  the  evolution 
of  the  muscular  system  in  the  vertebrates.  When  a  muscle 
normally  degenerates,  it  becomes  converted  into  connective  tissue, 
and  many  of  the  strong  aponeurotic  sheets  which  occur  in  the 
body  owe  their  origin  to  this  process.  Thus,  for  example,  the 
aponeurosis  connecting  the  occipital  and  frontal  portions  of  the 
occipito-frontalis  is  formed  in  this  way  and  is  muscular  in  such 
forms  as  the  lower  monkeys,  and  a  good  example  is  also  to  be 
found  in  the  aponeurosis  which  occupies  the  interval  between 
the  superior  and  inferior  serrati  postici,  these  two  muscles  being 
continuous  in  lower  forms.  The  strong  lumbar  aponeurosis  and 
the  aponeuroses  of  the  oblique  and  transverse  muscles  of  the 
abdomen  are  also  good  examples. 

Indeed,  in  comparing  a  mammal  with  a  member  of  one  of  the 
lower  classes  of  vertebrates,  the  greater  amount  of  connective 
tissue  compared  with  the  amount  of  muscular  tissue  in  the  former 
is  very  striking,  the  inference  being  that  these  connective- tissue 
structures  (fasciae,  aponeuroses,  ligaments)  represent  portions 
of  the  muscular  tissue  of  the  lower  form  (Bardeleben).  Many 
of  the  accessory  ligaments  occurring  in  connection  with  diarthro- 
dial  joints  apparently  owe  their  origin  to  a  degeneration  of 
muscle  tissue,  the  fibular  lateral  Hgament  of  the  knee-joint, 
for  instance,  being  probably  a  degenerated  portion  of  the 
peroneus  longus,  while  the  sacro-tuberous  ligament  appears  to 


202  THE    TRUNK    MUSCULATURE 

stand  in  a  similar  relation  to  the  long  head  of  the  biceps  femoris 
(Sutton). 

6.  Finally,  there  may  be  associated  with  any  of  the  first  four 
processes  a  change  in  the  direction  of  the  muscle-fibers.  The 
original  antero-posterior  direction  of  the  fibers  is  retained  in  com- 
paratively few  of  the  adult  muscles  and  excellent  examples  of 
the  process  here  referred  to  are  to  be  found  in  the  intercostal 
muscles  and  the  muscles  of  the  abdominal  walls. 

It  would  occupy  too  much  space  in  a  work  of  this  kind  to  con- 
sider in  detail  the  history  of  each  one  of  the  skeletal  muscles  of 
the  human  body,  but  a  statement  of  the  general  plan  of  their 
development  will  not  be  out  of  place.  For  convenience  the 
entire  system  may  be  divided  into  three  portions — the  cranial, 
trunk  and  limb  musculature;  and  of  these,  the  trunk  musculature 
may  first  be  considered. 

The  Trunk  Musculature.^It  has  already  been  seen  (p.  85) 
that  the  myotomes  at  first  occupy  a  dorsal  position,  becoming 
prolonged  ventrally  as  development  proceeds  so  as  to  overlap  the 
somatic  mesoderm,  until  those  of  opposite  sides  come  into  contact 
in  the  mid- ventral  line.  Before  this  is  accomplished,  however,  a 
longitudinal  splitting  of  each  myotome  occurs,  whereby  there  is 
separated  off  a  dorsal  portion  which  gives  rise  to  a  segment  of  the 
dorsal  musculature  of  the  trunk  and  is  supplied  by  the  ramus 
dorsalis  of  its  corresponding  spinal  nerve.  In  the  lower  vertebrates 
this  separation  of  each  of  the  trunk  myotomes  into  a  dorsal  and 
ventral  portion  is  much  more  distinct  in  the  adult  than  it  is  in 
man,  the  two  portions  being  separated  by  a  horizontal  plate  of 
connective  tissue  extending  the  entire  length  of  the  trunk  and 
being  attached  by  its  inner  edge  to  the  transverse  processes  of 
the  vertebrae,  while  peripherally  it  becomes  continuous  with  the 
connective  tissue  of  the  dermis  along  a  fine  known  as  the  lateral 
line.  In  man  the  dorsal  portion  is  also  much  smaller  in  proportion 
to  the  ventral  portion  than  in  the  lower  vertebrates.  From  this 
dorsal  portion  of  the  myotomes  are  derived  the  muscles  belonging 
to  the  three  deepest  layers  of  the  dorsal  musculature,  the  more 
superficial  layers  being  composed  of  muscles  belonging  to    the 


THE    TRUNK    MUSCULATURE  203 

limb  system.  Further  longitudinal  and  tangential  divisions  and 
a  fusion  of  successive  myotomes  bring  about  the  conditions  which 
obtain  in  the  adult  dorsal  musculature. 

While  the  myotomes  are  still  some  distance  from  the  mid- 
ventral  line  another  longitudinal  division  affects  their  ventral 
edges  (Fig.  122),  portions  being  thus  separated  which  later  fuse 
more  or  less  perfectly  to  form  longitudinal  bands  of  muscle,  those 


Pig.  122. — Embryo  of  13  mm.  showing  the  Formation  of  the  Rectus  Muscle. — 

(Mall.) 

of  opposite  sides  being  brought  into  apposition  along  the  mid- 
ventral  line  by  the  continued  growth  ventrally  of  the  myotomes. 
In  this  way  are  formed  the  rectus  and  pyramidalis  muscles  of  the 
abdomen  and  the  depressors  of  the  hyoid  bone,  the  genio-hyoid 
and  genio-glossus*  in  the  neck  region.     In  the  thoracic  region  this 

*  This  muscle  is  supplied  by  the  hypoglossal  nerve,  but  for  the  present  purpose  it 
is  convenient  to  regard  this  as  a  spinal  nerve,  as  indeed  it  primarily  is. 


204  THE   TRUNK    MUSCULATURE 

rectus  set  of  muscles,  as  it  may  be  termed,  is  not  represented 
except  as  an  anomaly,  its  absence  being  probably  correlated  with 
the  development  of  the  sternum  in  this  region. 

The  lateral  portions  of  the  myotomes  which  intervene  between 
the  dorsal  and  rectus  muscles  divide  tangentially,  producing  from 
their  dorsal  portions  in  the  cervical  and  lumbar  regions  muscles, 
such  as  the  longus  capitis  and  colli  and  the  psoas,  which  lie  be- 
neath the  vertebral  column  and  hence  have  been  termed  hypo- 
skeletal  muscles  (Huxley).  More  ventrally  three  sheets  of  mus- 
cles, lying  one  above  the  other,  are  formed,  the  fibers  of  each  sheet 
being  arranged  in  a  definite  direction  differing  from  that  found  in 
the  other  sheets.  In  the  abdomen  there  are  thus  formed  the  two 
oblique  and  the  transverse  muscles  together  with  the  quadratus 
lumborum,  in  the  thorax  the  intercostals  and  the  transversus 
thoracis,  while  in  the  neck  these  portions  of  some  of  the  myo- 
tomes disappear,  those  of  the  remainder  giving  rise  to  the  scaleni 
muscles,  portions  of  the  trapezius  and  sternomastoid  (Bolk), 
and  possibly  the  hyoglossus  and  styloglossus.  In  the  abdominal 
region,  and  to  a  considerable  extent  in  the  neck  also,  the  various 
portions  of  myotomes  fuse  together,  but  in  the  thorax  they  retain 
in  the  intercostals  their  original  distinctness,  being  separated  by 
the  ribs. 

The  table  on  p.  205  will  show  the  relation  of  the  various  trunk 
muscles  to  the  portions  of  the  myotomes. 

The  intimate  association  between  the  pelvic  girdle  and  the 
axial  skeleton  brings  about  extensive  modifications  of  the  posterior 
trunk  myotomes.  So  far  as  their  dorsal  portions  are  concerned 
probably  all  these  myotomes  as  far  back  as  the  fifth  sacral  are 
represented  in  the  sacro-spinalis,  but  the  ventral  portions  from  the 
first  lumbar  myotome  onward  are  greatly  modified.  The  last 
myotome  taking  part  in  the  formation  of  the  rectus  abdominis 
is  the  twelfth  thoracic  and  the  last  to  be  represented  in  the  lateral 
musculature  of  the  abdomen  is  the  first  lumbar,  the  ventral  por- 
tions of  the  remaining  lumbar  and  of  the  first  and  second  sacral 
myotomes  having  disappeared. 

The  ventral  portions  of  the  third  and  fourth  sacral  myotomes 


THE   TRUNK   MUSCULATURE 


205 


2o6 


THE    CRANIAL   MUSCULATURE 


are  represented,  however,  by  the  levator  ani  and  coccygeus,  and 
are  the  last  myotomes  which  persist  as  muscles  in  the  human  body, 
although  traces  of  still  more  posterior  myotomes  are  to  be  found 
in  muscles  such  as  the  curvator  coccygis  sometimes  developed  in 
connection  with  the  coccygeal  vertebrae. 

The  perineal  muscles  and  the  external  sphincter  ani  are  also 
developments  of  the  third  and  fourth  (and  second)  sacral  myo- 
tomes. At  a  time  when  the  cloaca  (see  p.  282)  is  still  present,  a 
sheet  of  muscles  lying  close  beneath  the  integument  forms  a 


Fig.  123. — Perineal  Region  of  Embryos  of  (A)  Two  Months  and  (B)  Four  to 

Five  Months,  showing  the  Development  of  the  Perineal  Muscles. 
dc,   Nervus  dorsalis  clitoridis;  p,  pudendal  nerve;  sa,  sphincter  ani;  sc,  sphincter 
cloacae;  sv,  sphincter  vaginas. — (Popowsky.) 

sphincter  around  its  opening  (Fig.  123).  On  the  development  of 
the  partition  which  divides  the  cloaca  into  rectal  and  urinogenital 
portions,  the  sphincter  is  also  divided,  its  more  posterior  por- 
tion persisting  as  the  external  sphincter  ani,  while  the  anterior 
part  gradually  differentiates  into  the  various  perineal  muscles 
(Popowsky). 

The  Cranial  Musculature.— ^As  was  pointed  out  in  an  earlier 
chapter,  the  existence  of  distinct  mesodermic  somites  has  not  yet 
been  completely  demonstrated  in  the  head  of  the  human  embryo, 
but  in  lower  forms,  such  as  the  elasmobranch  fishes,  they  are 
clearly  distinguishable,  and  it  may  be  supposed  that  their  indis- 
tinctness in  man  is  a  secondary  condition.  Exactly  how'many  of 
these  somites  are  represented  in  the  mammalian  head  it  is  im- 
possible to  say,  but  it  seems  probable,  from  comparison  with  lower 


THE    CRANIAL    MUSCULATURE  207 

forms,  that  there  is  a  considerable  number.  The  majority  of 
them,  however,  early  undergo  degeneration,  and  in  the  adult 
condition  only  three  are  recognizable,  two  of  which  are  praeoral  m 
position  and  one  postoral.  The  myotomes  of  the  anterior  praeoral 
segment  give  rise  to  the  muscles  of  the  eye  supplied  by  the  third 
cranial  nerve,  those  of  the  posterior  one  furnish  the  superior 
oblique  muscles  innervated  by  the  fourth  nerve,  while  from  the 
postoral  myotomes  the  lateral  recti,  supplied  by  the  sixth  nerve, 
are  developed.  The  muscles  suppHed  by  the  hypoglossal  nerve 
are  also  derived  from  myotomes,  but  they  have  already  been 
considered  in  connection  with  the  trunk  musculature. 

The  remaining  muscles  of  the  head  differ  from  the  voluntary 
muscles  of  the  trunk  in  the  fact  that  they  are  derived  from  the 
branchiomeres  formed  by  the  segmentation  of  the  cephalic  ventral 
mesoderm.  These  muscles,  therefore,  are  not  to  be  regarded  as 
equivalent  to  the  myotomic  muscles  if  their  embryological  origin 
is  to  be  taken  as  a  criterion  of  equivalency,  and  it  would  seem, 
from  the  fact  that  they  are  innervated  by  nerves  fundamentally 
distinct  from  those  which  supply  the  myotomic  muscles,  that  this 
criterion  is  a  good  one.  They  must  be  regarded,  therefore,  as 
belonging  to  a  special  category,  and  may  be  termed  branchiomeric 
muscles  to  distinguish  them  from  the  myotomic  set. 

If  their  embryological  origin  be  taken  as  a  basis  for  homology,  it  is 
clear  that  they  should  be  regarded  as  equivalent  to  the  muscles  derived 
from  the  ventral  mesoderm  of  the  trunk,  and  these,  as  has  been  seen, 
are  the  non-striated  muscles  associated  with  the  viscera,  among  which 
may  be  included  the  striated  heart  muscle.  At  first  sight  this  homology 
seems  decidedly  strained,  chiefly  because  long-continued  custom  has 
regarded  the  histological  and  physiological  peculiarities  of  striated  and 
non-striated  muscle  tissue  as  fundamental.  It  may  be  pointed  out, 
however,  that  the  branchiomeric  muscles  are,  strictly  speaking,  visceral 
muscles,  and  indeed  give  rise  to  muscle  sheets  (the  constrictors  of  the 
pharynx)  which  surround  the  upper  or  pharyngeal  portion  of  the  di- 
gestive tract.  It  is  possible,  then,  that  the  homology  is  not  so  strained 
as  might  appear,  but  further  discussion  of  it  may  profitably  be  de- 
ferred until  the  cranial  nerves  are  under  consideration. 

The  skeleton  of  the  first  branchial  arch  becomes  converted 
partly  into  the  jaw  apparatus  and  partly  into  auditory  ossicles, 


208 


THE    CRANIAL    MUSCULATURE 


Fig.  124. — Head  of  Embryos  (A)  of  Two  Months  and  (B)  of  Three  Months 

SHOWING  THE  EXTENSION  OF  THE  SEVENTH   NeRVE  UPON  THE  PaCE. —  {PopOWSky.) 


THE    CRANIAL   MUSCULATURE  209 

and  the  muscles  derived  from  the  corresponding  branchiomere 
become  the  muscles  of  mastication  (the  temporal,  masseter,  and 
pterygoids),  the  mylohyoid,  anterior  belly  of  the  digastric,  the— 
tensor  veli  palatini  and  the  tensor  tympani.  The  nerve  which 
corresponds  to  the  first  branchial  arch  is  the  trigeminus  or  fifth, 
and  consequently  these  various  muscles  are  supplied  by  it. 

The  second  arch  has  corresponding  to  it  the  seventh  nerve,  and 
its  musculature  is  partly  represented  by  the  stylohyoid  and 
posterior  belly  of  the  digastric  and  by  the  stapedius  muscle  of  the 
middle  ear.  From  the  more  superficial  portions  of  the  branchio- 
mere, however,  a  sheet  of  tissue  arises  which  gradually  extends 
upward  and  downward  to  form  a  thin  covering  for  the  entire 
head  and  neck,  its  lower  portion  giving  rise  to  the  platysma  and 
the  nuchal  fascia  which  extends  backward  from  the  dorsal  border 
of  this  muscle,  while  its  upper  parts  become  the  occipito-frontalis 
and  the  superficial  muscles  of  the  face  (the  muscles  of  expression), 
together  with  the  fasciae  which  unite  the  various  muscles  of  this 
group.  The  extension  of  the  platysma  sheet  of  muscles  over  the 
face  is  well  shown  T^y  the  development  of  the  branches  of  the  facial 
nerve  which  supply  it  (Fig.  124). 

The  degeneration  of  the  upper  part  of  the  third  arch  produces 
a  shifting  forward  of  one  of  the  muscles  derived  from  its  branchio- 
mere, the  stylopharyngeus  arising  from  the  base  of  the  styloid 
process.  The  innervation  of  this  muscle  by  the  ninth  nerve  indi- 
cates, however,  its  true  significance,  and  since  fibers  of  this  nerve 
of  the  third  arch  also  pass  to  the  constrictor  muscles  of  the 
pharynx,  a  portion  of  these  must  also  be  regarded  as  having  arisen 
from  the  third  branchiomere. 

The  cartilages  of  the  fourth  and  fifth  arches  enter  into  the 
formation  of  the  larynx  and  the  muscles  of  the  corresponding 
branchiomeres  constitute  the  muscles  of  the  larynx,  together  with 
the  remaining  portions  of  the  constrictors  of  the  pharynx  and  the 
muscles  of  the  soft  palate,  with  the  exception  of  the  tensor.  Both 
these  arches  have  branches  of  the  tenth  nerve  associated  with 
them  and  hence  this  nerve  supplies  the  muscles  named.  In  addi- 
tion, two  of  the  extrinsic  muscles  of  the  tongue,  the  glossopalatinus 

14 


2IO 


THE    CRANIAL   MUSCULATURE 


Tenth            Eleventh 

Trapezius. 

Sterno- 

mastoid. 

Constrictors 
of  pharynx 

(in  part). 

Pharyngo- 

palalinus. 

Levator  veli 

palatini. 

Musculus 

uvulae. 
Muscles  of 
the  larynx. 
Glosso-pal- 

atinus. 

Chrondro- 

glossus. 

a 

Stylo-pha- 

ryngeus. 

Constrictors 

of  pharynx 
(in  part). 

Stylohyoid. 
Digastric 

(posterior 
belly). 

Stapedius. 

Platysma. 
Occipito- 
frontalis. 

Muscles  of 

expression. 

^ 

« 
c^ 

3^ 

A 

1 

Temporal. 

Masseter. 
Pterygoids. 
Mylohyoid. 

Digastric 

(anterior) 

belly). 
Tensor  veli 

palatini. 
Tensor 

tympani. 

43 

(2 

2  oJ 

I 

Superior  ] 
Inferior       recti. 
Medial    J 
Inferior  oblique. 

1 

o 
I 

s 

'C 

1 

<-> 

c 

>-< 
pq 

3 

s 

THE    LIMB    MUSCLES  211 

and  chondroglossus,  belong  to  the  fourth  or  fifth  branchiomere, 
although  the  remaining  muscles  of  this  physiological  set  are  myo- 
tomic  in  origin. 

Finally,  portions  of  two  other  muscles  should  probably  he  in- 
cluded in  the  list  of  branchiomeric  muscles,  these  muscles  being 
the  trapezius  and  sternomastoid.  It  has  already  been  seen  that 
they  are  partly  derived  from  the  cervical  myotomes,  but  they  are 
also  innervated  in  part  by  the  spinal  accessory,  and  since  this 
nerve  is  really  a  special  portion  of  the  motor  root  of  the  vagus  the 
muscles  supphed  by  it  should  be  regarded  as  branchiomeric  in  origin. 

The  table  on  p.  210  shows  the  relations  of  the  various  cranial 
muscles  to  the  myotomes  and  branchiomeres,  as  well  as  to  the 
motor  cranial  nerves. 

The  Limb  Muscles. — It  has  been  customary  to  regard  the  limb 
muscles  as  derivatives  of  certain  of  the  myotomes,  these  structures 
in  their  growth  ventrally  in  the  trunk  walls  being  supposed  to  pass 
out  upon  the  postaxial  surface  of  the  limb  buds  and  loop  back 
again  to  the  trunk  along  the  praeaxial  surface,  each  myotome  thus 
giving  rise  to  a  portion  of  both  the  dorsal  and  the  ventral  muscu- 
lature of  the  limb.  This  view  has  not,  however,  been  verified  by 
direct  observation  of  an  actual  looping  of  the  myotomes  over  the 
axis  of  the  limb  buds;  indeed,  on  the  contrary,  the  limb  muscles 
have  been  found  to  develop  from  the  cores  of  mesenchyme  which 
form  the  axes  of  the  limb  buds  and  from  which  the  limb  skeleton  is 
also  developed,  and,  furthermore,  these  axial  cores  can  be  traced 
back  to  an  origin  from  the  unsegmented  ventral  mesoderm,  the 
adjacent  myotomes  having  apparently  no  part  in  their  formation. 
It  seems  proper,  therefore,  to  regard  the  limb  musculature  as  be- 
longing to  a  different^embryological  category  from  the  axial  myo- 
tomic  muscles,  just  as  was  the  case  of  the  branchiomeric  musculature. 

The  strongest  evidence  in  favor  of  a  myotomic  origin  of  the 
limb  muscles  is  that  furnished  by  their  nerve  supply,  this  present- 
ing a  distinctly  segmental  arrangement.  This  does  not  necessarily 
imply,  however,  a  corresponding  primarily  metameric  arrangement 
of  the  muscles,  any  more  than  the  pronouncedly  segmental  ar- 
rangement of  the  cutaneous  nerves  implies  a  primary  metamerism 


212 


THE   LIMB    MUSCLES 


of  the  dermis  (see  p.  145).  It  may  mean  only  that  the  nerves, 
being  segmental,  retain  their  segmental  relations  to  one  another 
even  in  their  distribution  to  non-metameric  structures,  and  that, 
consequently,  the  limb  musculature  is  supplied  in  succession  from 
one  border  of  the  limb  bud  to  the  other  from  succeeding  nerve 
roots. 

From  this  segmentally  arranged  innervation  it  is  possible  to 
recognize  in  the  limb  buds  a  series  of  parallel  bands  of  muscle 


tr.d 


^.ir 


vm 


Fig.  125. — Diagram  of  a  Segment  of  the.  Body  and  Limb. 
hi.  Axial  blastema;  dm,  dorsal  musculature  of  trunk;  rl,  nerve  to  limb;  s,  septum 
between  dorsal  and  ventral  trunk  musculature;  sir.  d,  dorsal  layer  of  limb  muscula- 
ture; tr.d  and  Ir.v,  dorsal  and  ventral  divisions  of  a  spinal  nerve;  vm,  ventral  muscu- 
lature of  the  trunk. — (Kollmann.) 

tissue,  extending  longitudinally  along  the  bud  and  each  supplied 
by  a  definite  segmental  nerve.  And  furthermore,  corresponding 
to  each  band  upon  the  ventral  (praeaxial)  surface  of  the  limb  bud, 
there  is  a  band  similarly  innervated  upon  the  dorsal  (postaxial) 
surface,  since  the  fibers  which  pass  to  the  limb  from  each  nerve 
root  sooner  or  later  arrange  themselves  in  praeaxial  and  postaxial 
groups  as  is  shown  in  the  diagram  Fig.  125.  The  first  nerve  which 
enters  the  limb  bud  lies  along  its  anterior  border,  and  consequently 


THE    LIMB    MUSCLES 


213 


the  muscle  bands  which  are  supplied  by  it  will,  in  the  adult,  lie 
along  the  outer  side  of  the  arm  and  along  the  inner  side  of  the  leg, 
in  consequence  of  the  rotation  in  opposite  directions  which  the 
Hmbs  undergo  during  development  (see  p.  104). 


Fig.  126. — External  Surface  of  the  Os  Innominatum  showing  the  Attach- 
ment OF  Muscles  and  the  Zones  Supplied  by  the  Various  Nerves. 
12,  Twelfth  thoracic  nerve;  /  to  V,  lumbar  nerves;  i  and  2,  sacral  nerves. — (Bolk.) 

The  first  nerve  which  supplies  the  muscle  attached  to  the 
dorsum  of  the  ilium  is  the  second  lumbar,  and  following  that  there 
come  successively  from  before  backward  the  remaining  lumbar 


214 


THE    LIMB    MUSCLES 


and  the  first  and  second  sacral  nerves.  The  arrangement  of  the 
muscle  bands  supplied  by  these  nerves  and  the  muscles  of  the 
adult  to  which  they  contribute  may  be  seen  from  Fig.  126. 
What  is  shown  there  is  only  the  upper  portions  of  the  postaxial 


Fig.  127. — Sections  through  (A)  the  Thigh  and  (B)  -the  Calf  showing 
THE  Zones  Supplied  by  the  Nerves.  The  Nerves  are  Numbered  in  Con- 
tinuation with  the  Thoracic  Series. — (A,  after  Bolk.) 


bands,  their  lower  portions  extending  downward  on  the  anterior 
surface  of  the  leg.  Only  the  sacral  bands,  however,  extend 
throughout  the  entire  length  of  the  limb  into  the  foot,  the  second 
lumbar  band  passing  down  only  to  about  the  middle  of  the  thigh, 


THE    LIMB    MUSCLES  21$ 

the  third  to  about  the  knee,  the  fourth  to  about  the  middle  of  the 
crus  and  the  fifth  as  far  as  the  base  of  the  fifth  metatarsal  bone, 
and  the  same  is  true  of  the  corresponding  praeaxial  bands,  which 
descend  from  the  ventral  surface  of  the  os  coxae  (innominatum) 
along  the  inner  and  posterior  surfaces  of  the  leg  to  the  same  points. 
The  first  and  second  sacral  bands  can  be  traced  into  the  foot,  the 
first  giving  rise  to  the  musculature  of  its  inner  side  and  the  second 
to  that  of  its  outer  side,  the  praeaxial  bands  forming  the  plantar 
musculature,  while  the  postaxial  ones  are  upon  the  dorsum  of  the 
foot  as  a  result  of  the  rotation  which  the  limb  has  undergone. 

In  a  transverse  section  through  a  limb  at  any  level  all  the 
muscle  bands,  both  praeaxial  and  postaxial,  which  descend  to  that 
level  will  be  cut  and  will  lie  in  a  definite  succession  from  one  border 
of  the  limb  to  the  other,  as  is  seen  in  Fig.  127.  In  the  differentia- 
tion of  the  individual  muscles  which  proceeds  as  the  nerves  extend 
from  the  trunk  into  the  axial  mesenchyme  of  the  limb,  the  muscle 
bands  undergo  modifications  similar  to  those  already  described  as 
occurring  in  the  trunk  myotomes.  Thus,  there  has  evidently 
been  a  longitudinal  splitting  of  the  original  praeaxial  muscle  mass 
to  form  the  various  muscles  of  the  back  of  the  thigh;  the  soleus  and 
gastrocnemius  represent  deep  and  superficial  layers  formed  from 
the  same  bands  by  a  horizontal  (tangential)  splitting;  these  same 
muscles  contain  a  portion  of  the  second  sacral  band  which  overlaps 
muscles  composed  only  of  higher  bands;  and  the  intermuscular 
septum  between  the  peroneus  brevis  and  the  flexor  hallucis  longus 
represents  a  portion  of  the  third  sacral  band  which  has  degenerated 
into  connective  tissue. 

A  similar  arrangement  occurs  in  the  bands  which  are  to  be 
recognized  in  the  musculature  of  the  upper  Hmb.  These  are  sup- 
plied by  the  fourth,  fifth,  sixth,  seventh  and  eighth  cervical  and 
the  first  thoracic  nerves,  and  only  those  suppHed  by  the  eighth 
cervical  and  the  first  thoracic  nerves  extend  as  far  as  the  tips  of 
the  fingers.  The  arrangement  of  the  bands  in  the  upper  part  of 
the  brachium  may  be  seen  from  Fig.  128,  in  connection  with  which 
it  must  be  noted  that  the  fourth  cervical  band  does  not  extend 
down  to  the  level  at  which  the  section  is  taken  and  that  the 


2l6 


THE    LIMB    MUSCLES 


praeaxial  band  of  the  eighth  cervical  nerve  and  both  the  praeaxial 
and  postaxial  bands  of  the  first  thoracic  are  represented  only  by 
connective  tissue  in  this  region. 

In  another  sense  than  the  longitudinal  one  there  is  a  division 
of  the  limb  musculature  into  more  or  less  definite  areas,  namely, 
in  a  transverse  direction  in  accordance  with  the  jointing  of  the 
skeleton.  Thus,  there  may  be  recognized  a  group  of  muscles  which 
pass  from  the  axial  skeleton  to  the  limb  girdle,  another  from  the 
limb  girdle  to  the  brachium  or  thigh,  another  from  the  brachium 


Fig.  128. — Section  through  the  Upper  Part  of  the  Arm  showing  the  Zones 

Supplied  by  the  Nerves. 
5v  to  yv,  Ventral  branches;  5^  to  8d,  dorsal  branches  of  the  cervical  nerves. — (Bolk.) 

or  thigh  to  the  antibrachium  or  crus,  another  from  the  anti- 
brachium  or  crus  to  the  carpus  or  tarsus,  and  another  from  the 
carpus  or  tarsus  to  the  digits.  This  transverse  segmentation 
if  it  may  be  so  termed  is  not,  however,  perfectly  definite,  many 
muscles,  even  in  the  lower  vertebrates,  passing  over  more  than 
one  joint,  and  in  the  mammalia,  especially,  it  is  further  obscured 
by  secondary  migrations,  by  the  partial  degeneration  of  muscles 
and  by  an  end  to  end  union  of  primarily  distinct  muscles. 

The  latissimus  dorsi,  serratus  anterior  and  pectoral  muscles 
are  all  examples  of  a  process  of  migration  as  is  shown  by  their 
innervation  from  cervical  nerves,  as  well  as  by  the  actual  migration 
which  has  been  traced  in  the  developing  embryo  (Mall,  Lewis). 


THE    LIMB    MUSCLES  21 7 

In  the  lower  limb  evidences  of  migration  may  be  seen  in  the  femoral 
head  of  the  biceps,  comparative  anatomy  showing  this  to  be  a 
derivative  of  the  gluteal  set  of  muscles  which  has  secondarily  be- 
come attached  to  the  femur  and  has  associated  itself  with  a  prae- 
axial  muscle  to  form  a  compound  structure.  An  appearance  of 
migration  may  also  be  produced  by  a  muscle  making  a  secondary 
attachment  below  its  original  origin  or  above  the  insertion  and  the 
upper  or  lower  part,  as  the  case  may  be,  then  degenerating  into 
connective  tissue.  This  has  been  the  case  with  the  peroneus 
longus,  which,  in  the  lower  mammals,  has  a  femoral  origin,  but 
has  in  man  a  new  origin  from  the  fibula,  its  upper  portion  being 
represented  by  the  fibular  lateral  ligament  of  the  knee-joint.  So 
too  the  pectoralis  minor  is  primarily  inserted  into  the  humerus, 
but  it  has  made  a  secondary  attachment  to  the  coracoid  process, 
its  distal  portion  forming  a  coraco-humeral  ligament. 

The  comparative  study  of  the  flexor  muscles  of  the  anti- 
brachial  and  crural  regions  has  yielded  abundant  evidence  of  ex- 
tensive modifications  in  the  differentiation  of  the  limb  muscles. 
In  the  tailed  amphibia  these  muscles  are  represented  by  a  series 
of  superposed  layers,  the  most  superficial  of  which  arises  from 
the  humerus  or  femur,  while  the  remaining  ones  take  their  origin 
from  the  ulna  or  fibula  and  are  directed  distally  and  laterally  to 
be  inserted  either  into  the  palmar  or  plantar  aponeurosis,  or,  in 
the  case  of  the  deeper  layers,  into  the  radius  (tibia)  or  carpus 
(tarsus).  In  the  arm  of  the  lower  mammaha  the  deepest  layer 
becomes  the  pronator  quadratus,  the  lateral  portions  of  the  super- 
ficial layer  are  the  flexor  carpi  ulnaris  and  the  flexor  carpi  radialis, 
while  the  intervening  layers,  together  with  the  median  portion 
of  the  superficial  one,  assuming  a  more  directly  longitudinal 
direction,  fuse  to  form  a  common  flexor  mass  which  acts  on  the 
digits  through  the  palmar  aponeurosis.  From  this  latter  structure 
and  from  the  carpal  and  metacarpal  bones  five  layers  of  palmar 
muscles  take  origin.  The  radial  and  ulnar  portions  of  the  most 
superficial  of  these  become  the  flexor  pollicis  brevis  and  abductor 
pollicis  brevis  and  the  abductor  quinti  digiti,  while  the  rest  of  the 
layer  degenerates  into  connective  tissue,  forming  tendons  which 


2l8 


THE   LIMB    MUSCLES 


pass  to  the  four  ulnar  digits.  Gradually  superficial  portions  of  the 
antibrachial  flexor  mass  separate  off,  carrying  with  them  the  layers 
of  the  palmar  aponeurosis  from  which  the  tendons  representing 
the  superficial  layer  of  the  palmar  muscles  arise,  and  they  form 
with  these  the  flexor  digitorum  sublimis.     The  deeper  layers  of 


Pig.  129. — Transverse  sections  through  (A)  the  forearm  and  (B)  the  hand  show- 
ing the  arrangement  of  the  layers  of  the  flexor  muscles.  The  superficial  layer  is 
shaded  horizontally,  the  second  layer  vertically,  the  third  obliquely  to  the  left,  the 
fourth  vertically,  and  the  fifth  obliquely  to  the  right.  AbM,  abductor  digiti  quinti; 
AdP,  adductor  pollicis;  BR,  brachio-radialis ;  ECD,  extensor  digtorum  communis; 
ECU,  extensor  carpi  ulnaris;  EI,  extensor  indicis;  EMD,  extensor  digiti  quinti;  EMP, 
abductor  pollicis  longus;  ERB,  extensor  carpi  radialis  brevis;  FCR,  flexor  carpi 
radialis;  FCU,  flexor  carpi  ulnaris;  FLP,  flexor  pollicis  longus;  FM,  flexor  digiti 
quinti  brevis;  FP,  flexor  digitorum  profundus;  FS,  flexor  digitorum  sublimis;  ID 
interossei  dorsales;  IV,  interossei  volares;  L,  lumbricales;  OM,  opponens  digiti  quinti 
PL,  palmaris  longus;  PT,  pronator  teres;  R,  radius;  U,  ulna;  II-V,  second  to  fifth 
metacarpal. 

of  the  antibrachial  flexor  mass  become  the  flexor  digitorum 
profundus  and  the  flexor  pollicis  longus  (Fig.  129,  A),  and  retain 
their  connection  with  the  deeper  layers  of  the  palmar  aponeurosis 
which  form  their  tendons;  and  since  the  second  layer  of  the  palmar 
muscles  takes  origin  from  this  portion  of  the  aponeurosis  it  be- 


THE    LIMB    MUSCLES 


219 


comes  the  lumbrical  muscles,  arising  from  the  profundus  tendons 
(Fig.  129,  B).  The  third  layer  of  palmar  muscles  becomes  the 
adductors  of  the  digits,  reduced  in  man  to  the  adductor  pollicis, 
while  from  the  two  deepest  layers  the  interossei  are  developed. 
Of  these  the  fourth  layer  consists  primarily  of  a  pair  of  slips  cor- 
responding to  each  digit,  while  the  fifth  is  represented  by  a  series 
of  muscles  which  extend  obliquely  across  between  adjacent  meta- 
carpals.    With  these  last  muscles  certain  of  the  fourth  layer  slips 


Fig.  30. — Transverse  sections  through  {A)  the  crus  and  (B)  the  foot,  showing 
the  arrangement  of  the  layers  of  the  flexor  muscles.  The  shading  has  the  same  sig- 
nificance as  in  the  preceding  figure.  A  hH,  abductor  hallucis ;  A  bM,  abductor  minimi 
digiti;  AdH,  adductor  hallucis;  ELD,  extensor  longus  digitorum;  F,  fibula;  FBD 
flexor  brevis  digitorum;  FBH,  flexor  brevis  hallucis;  FBM,  flexor  brevis  minimi  digiti; 
FLD,  flexor  longus  digitorum;  G,  gastrocnemius;  ID,  interossei  dorsales;  IV,  inter- 
ossei ventrales;  L,  lumbricales;  P,  plantaris;  Fe,  peroneus  longus;  Po,  popliteus;  S, 
soleus;  T,  tibia;  TA,  tibialis  anticus;  TP,  tibialis  posticus;  I-V,  first  to  fifth  meta- 
tarsal. 

unite  to  form  the  dorsal  interossei,  while  the  rest  become  the  volar 
interossei. 

The  modifications  of  the  almost  identical  primary  arrange- 
ment in  the  crus  and  foot  are  somewhat  different.  The  super- 
ficial layer  of  the  crural  flexors  becomes  the  gastrocnemius  and 
plantaris  (Fig.  30,  A)  and  the  deepest  layer  becomes  the  popliteus 
and  the  interosseous  membrane.  The  second  and  third  layers 
unite  to  form  a  common  mass  which  is  inserted  into  the  deeper 
layers  of  the  plantar  aponeurosis  and  later  differentiates  into  the 
soleus  and  the  long  digital  flexor,  the  former  shifting  its  insertion 
from  the  plantar  aponeurosis  to  the  os  calcis,  while  the  flexor 


220  LITERATURE 

retains  its  connection  with  the  deeper  layers  of  the  aponeurosis, 
these  separating  from  the  superficial  layer  to  form  the  long  flexor 
tendons.  The  fourth  layer  assumes  a  longitudinal  direction  and 
becomes  the  tibialis  posterior  and  the  flexor  hallucis  longus  and 
partly  retains  its  original  oblique  direction  and  its  connection 
with  the  deep  layers  of  the  plantar  aponeurosis,  becoming  the 
quadratus  plantse.  In  the  foot  (Fig.  129,  B)  the  superficial 
layer  persists  as  muscular  tissue,  forming  the  abductors,  the  flexor 
digitorum  brevis  and  the  medial  head  of  the  flexor  hallucis  brevis, 
the  second  layer  becomes  the  lumbricales,  and  the  third  the  lateral 
head  of  the  flexor  hallucis  brevis  and  the  abductor  hallucis, 
while  the  fourth  and  fifth  layers  together  form  the  interossei,  as 
in  the  hand,  the  flexor  quinti  digiti  brevis  really  belonging  to  that 
group  of  muscles. 

LITERATURE 

C.  R.  Bardeen  and  W.  H.  Lewis:  "Development  of  the  Limbs,  Body- wall,  and 

Back  in  Man,"  The  American  Journal  of  Anal.,  i,  1901. 
K.   Bardeleben:  "Muskel  und  Fascia"  Jenaische  Zeitschr.  Jiir  Naturwissensch., 

XV,  1882. 
L.  BoLK:  "Beziehungen  zwischen  Skelett,  Muskulatur  und  Nerven  der  Extremitaten, 

dargelegt  am  Beckengiirtel,  an  dessen  Muskulatur    sowie  am  Plexus   lumbo- 

sacralis,"  Morphol.  Jahrhuch,  xxi,  1894. 
L.  Bolk:  "  Rekonstruktion  der  Segmentirung  der  Gliedmassenmuskulatur  dargelegt 

an  den  Muskeln  des  Oberschenkels  und  des  Schultergiirtels,"  Morphol.  Jahrhuch, 

xxn,  1895. 
L.  Bolk:  "Die  Sklerozonie  des  Humerus,"  Morphol.  Jahrhuch,  xxiii,  1896. 
L.    Bolk:  "Die    Segmentaldifferenzierung   des   menschlichen    Rumpfes  und  seiner 

Extremitaten,"  i,  Morphol.  Jahrhuch.  xxv,  1898. 
R.  Futamura:  "Ueber  die  Entwickelung  der  Facialismuskulatur  des  Menschen," 

Anat.  Hefte,  xxx,  1906. 
E.  Godlewski:  "Die  Entwicklung  des  Skelet-  und  Herzmuskelgewebes  der  Sauge- 

thiere,"  Archiv  Jiir  mikr.,  Anat.  lx,  1902. 
E.  Grafenberg:  "Die  Entwicklung  der  menschlichen  Beckenmuskulatur,"  Anat. 

Hefte,  xxin,  1904. 
W.  P.  Herringham:  "The  Minute  Anatomy  of  the  Brachial  Plexus,"  Proceedings 

of  the  Royal  Soc.  London,  xli,  1886. 
W.  H.  Lewis:  "The  Development  of  the  Arm  in  Man,"  Amer.  Journ.  of  Anat.,  i,  1902 
J.  B.  MacCallum:  "On  the  Histology  and  Histogenesis  of  the  Heart  Muscle-cell," 

A  nat.  A  nzeiger,  xiii,  1 89 7 . 
J.   B.  MacCallum:  "On  the  Histogenesis  of  the  Striated  Muscle-fiber  and  the 

Growth  of  the  Human  Sartorius  Muscle,"  Johns  Hopkins  Hospital  Bulletin,  1898. 


LITERATURE  221 

F.  P.  Mall:  "Development  of  the  Ventral  Abdominal  Walls  in  Man,"  Journ.  of 

Morphol,  XIV,  1898. 
Caroline  McGill:  "The  Histogenesis  of  Smooth  Muscle  in  the  Alimentary  Canal 

and  Respiratory  Tract  of  the  Pig,"  Internal.  Monatschr.  Anat.  und  Phys.,  xxiv, 

1907. 
J.  P.  McMurrich:  "The  Phylogeny  of  the  Forearm  Flexors,"  Amer.  Journ.  of 

Anat.,  II,  1903. 
J.  P.  McMurrich:  "The  Phylogeny  of  the  Palmar  Musculature,"  Amer.  Journ.  of 

Anat.,  II,  1903. 
J.  P.  McMurrich:  "The  Phylogeny  of  the  Crural  Flexors,"  Amer.  Journ.  of  Anat, 

IV,  1904.  • 

J.  P.  McMurrich:  "The  Phylogeny  of  the  Plantar  Musculature,"  Amer.  Journ.  of 

Anat.fYifigoy. 

A.  Meek:  "Preliminary  Note  on  the  Post-embryonal  History  of  Striped  Muscle- 

fibers  in  Mammalia,"  Anat.  Anzeiger,  xrv,  1898.     (See  also  Anat.  Anzeiger,xv, 
1899.) 

B.  MoRPURGo:  "Ueber    die    post-embryonale    Entwickelung    der    quergestreiften 

Muskel  von  weissen  Ratten,"  Anat.  Anzeiger,  xv,  1899, 
I.    Popowsky:  "Zur    Entwicklungsgeschichte    des    N.    facialis    beim    Menschen," 

Morphol.  Jahrhuch,  xxiu,  1896. 
I.  Popowsky:  "Zur  Entwickelungsgeschichte  der  Dammuskulatur  beim  Menschen," 

Anat.  Hefte,  xi,  1899. 
L.  Rethi:  "Der  peripheren  Verlauf  der  motorischen  Rachen-  und  Gaumennerven," 

Sitzungsher.  der  kais.   Akad.   Wissensch.   Wien.   Math.-Naturwiss.   Classe,   cii, 

1893. 

C.  S.  Sherrington:  "Notes  on  the  Arrangement  of  Some  Motor  Fibers  in  the 

Lumbo-sacral  Plexus,"  Journal  of  Physiol.,  xiii,  1892. 
J.  B.  Sutton:  "Ligaments,  their  Nature  and  Morphology,"  London,  1897. 


CHAPTER  IX 

THE  DEVELOPMENT  OF  THE  CIRCULATORY  AND 
LYMPHATIC  SYTEMS 

At  present  nothing  is  known  as  to  the  earliest  stages  of  de- 
velopment of  the  circulatory  system  in  the  human  embryo,  but 
it  may  be  supposed  that  they  resemble  in  their  fundamental  fea- 
tures what  has  been  observed  in  such  forms  as  the  rabbit  and  the 
chick.  It  will  be  convenient  to  consider  first  the  development 
of  the  first  blood-vessels  and  of  the  blood,  and  later  the  formation 
of  the  heart  and  principal  blood-vessels. 

In  the  rabbit  the  extension  of  the  mesoderm  from  the  embryo- 
nic region,  where  it  first  appears,  over  the  yolk-sac  is  a  gradual 
process,  and  it  is  in  the  more  peripheral  portions  of  the  layer  that 
the  blood-vessels  first  make  their  appearance.  They  can  be  dis- 
tinguished before  the  splitting  of  the  mesoderm  has  been  com- 
pleted, but  are  always  developed  in  that  portion  of  the  layer 
which  is  most  intimately  associated  with  the  yolk-sac,  and  conse- 
quently becomes  the  splanchnic  layer.  They  belong,  indeed,  to 
the  deeper  portion  of  that  layer,  that  nearest  the  endoderm  of  the 
yolk-sac,  and  so  characteristic  is  their  origin  from  this  portion  of 
the  layer  that  it  has  been  termed  the  angiohlast  and  has  been  held 
to  be  derived  from  the  endoderm  independently  of  the  meso- 
derm proper.  The  first  indication  of  blood-vessels  is  the  appear- 
ance in  the  peripheral  portion  of  the  mesoderm  of  cords  or  minute 
patches  of  spherical  cells  (Fig.  131,  A).  These  increase  in  size 
by  the  division  and  separation  of  the  cells  from  one  another  (Fig. 
131,  jB),  a  clear  fluid  appearing  in  the  intervals  which  separate 
them.  Soon  the  cells  surrounding  each  cord  arrange  themselves 
to  form  an  enclosing  wall,  and  the  cords,  increasing  in  size,  unite 
together  to  form  a  network  of  vessels  in  which  float  the  spherical 
cells  which  may  be  known  as  haemohlasts.     Viewed  from  the  sur- 

222 


DEVELOPMENT    OF   THE   BLOOD   VESSELS 


223 


face  at  this  stage  a  portion  of  the  vascular  area  of  the  mesoderm 
would  have  the  appearance  shown  in  Fig.  132,  revealing  a  dense 
network  of  canals  in  which,  at  intervals,  are  groups  of  haemo^ 
blasts  adherent  to  the  walls,  constituting  what  have  been  termed 
the  blood-islands,  while  in  the  meshes  of  the  network  unaltered 
mesoderm  cells  can  be  seen,  forming  the  so-called  substance-islands. 
Two  views  obtain  as  to  the  way  in  which  the  extension  of  the 
vascularization  process  from  the  extra  embryonic-regions  into  the 
body  of  the  embryo  takes  place.     In  one  the  angioblast  is  given 


Fig.  131. — Transverse  Sections  through  the  Area  Vasculosa  of  Rabbit 
Embryos  showing  the  Transformation  of  Mesoderm  Cells  into  the  Vascular 
Cords, 

Ec,  Ectoderm;  En,  endoderm;  Me,  mesoderm. — (van  der  Stricht.) 

the  status  of  an  additional  germ-layer,  distinct  from  the  mesoderm; 
it  is  a  specific  tissue,  set  apart  at  an  early  stage  of  the  develop- 
ment as  the  origin  of  all  the  vascular  apparatus,  endotheUum  and 
blood  elements,  of  the  embryo,  and  it  is  the  sole  source  of  this 
apparatus.  Hence  the  vascular  tissue  of  the  embryo  proper  is 
formed  by  the  extension  into  the  embryo  of  angioblastic  material 
from  the  extra-embryonic  regions,  the  vascular  endotheUum  is  a 
specific  tissue  and  the  embryonic  mesenchyme  has  no  part  in  its 
formation,  According  to  the  other  view  such  specificity  is  denied 
the  angioblast  and  it  is  held  that  vasifactive  tissue  may  be  formed 
locally  from  the  embryonic  mesenchyme;  the  angioblast  is  merely 


?24 


DEVELOPMENT  OF  THE  BLOOD  VESSELS 


extra-embryonic  mesenchyme  that  has  assumed  a  vasifactive  func- 
tion, and  by  processes  similar  to  those  shown  by  it  the  mesenchyme 
of  practically  any  part  of  the  embryo  proper  may  become  con- 
verted into  vascular  tissue,  producing  vascular  networks  which 

eventually  unite  directly  or  in- 
directly with  those  formed  by  the 
angioblast.  Briefly,  according  to 
this  view,  there  is  not  neces- 
sarily immediate  genetic  contin- 
uity of  all  the  vascular  endothe- 
lium, but  mesenchyme  cells  in  any 
region  of  the  body  may  become 
endothelium  and  conversely  en- 
dothelial cells  under  certain  condi- 
tions may  revert  to  mesenchyme. 
Whichever  of  these  views 
eventually  proves  to  be  correct 
the  end  result  is  that  the  blood 
vascular  system,  both  in  its  em- 
bryonic and  extra-embryonic  por- 
tions, consists  in  its  earlier  stages 
of  a  continuous  network  of  vessels 
Hned  with  endothelium  and  con- 
taining haemoblasts  formed  by 
the  multiplication  of  the  original 
haemoblasts  and  by  proliferation 
from  the  endothelial  cells  them- 
selves. Later,  enlargements  of 
the  network  develop  along  more 
or  less  definite  lines  to  form  the  heart,  the  arteries  and  veins,  other 
portions  of  it  persist  to  form  the  capillaries,  while  others  again 
disappear  entirely.  The  differentiation  of  blood-vessels  from  the 
network  on  the  surface  of  the  yolk-sac  of  a  rabbit  embryo  is  shown 
at  its  commencement  in  Fig.  133,  A  and  in  Fig.  133,  B  the  exten- 
sion of  the  differentiation  has  resulted  in  the  formation  of  a  sinus 
terminalis,  a  vitelline  artery  and  two  vitelline  veins. 


Fig.  132.  —  Surface  View  of  a 
Portion  of  the  Area  Vasculosa 
OF  A  Chick. 

The  vascular  network  is  represented 
by  the  shaded  portion.  Bi,  Blood- 
island;  Si,  substance-island. — (Disse.) 


DEVELOPMENT    OF   THE   BLOOD   VESSELS 


225 


In  the  human  embryo  the  sHght  development  of  the  yolk-sac 
and  the  increased  importance  of  the  chorion  in  the  nutrition  of 
the  embryo  have  apparently  led  to  a  reduction  in  the  development"" 
of  the  vitelline  network  and  an  acceleration  in  the  development  of 
the  chorionic  vessels  (see  p.  118),  but  otherwise  the  early  develop- 
ment of  the  blood-vascular  system  is  probably  similar  to  what 
has  been  described  for  the  rabbit. 


—^^''A 


Fig.  133. — The  Vascular  Areas-  of  Rabbit  Embryos.     In  B  the  Veins  are 
Represented  by  Black  and  the  Network  is  Omitted. — (von  Beneden  and  Julin.) 

It  is  to  be  noted  that  the  capillary  network  of  the  area  vascu- 
losa  consists  of  relatively  wide  anastomosing  spaces  whose  endo- 
thehal  lining  rests  directly  upon  the  substance  islands  (Fig.  131). 
In  certain  of  the  embryonic  organs,  notably  the  liver,  the  meta- 
nephros  and  the  heart,  when  these  have  become  vascularized, 
the  network  has  a  similar  character,  consisting  of  wide  anastomos- 
ing spaces  bounded  by  an  endothelium  which  rests  directly,  or 
almost  so,  upon  the  parenchyma  of  the  organ  (the  hepatic  cylin- 
ders, the  mesonephric  tubules,  or  the  cardiac  muscle  trabeculae) 
(Figs.  134  and  191,  ^).  To  this  form  of  capillary  the  term  sinusoid 
has  been  applied  (Minot),  and  it  appears  to  be  formed  by  the 
expansion  of  the  wall  of  a  previously  existing  blood-vessel,  which 
thus  moulds  itself,  as  it  were,  over  the  parenchyma  of  the  organ. 

15 


2  26  THE  FORMATION  OF  THE  BLOOD 

The  true  capillaries,  on  the  other  hand,  are  more  definitely  tubular 
in  form,  are  usually  imbedded  in  mesench\Tnatous  connective 
tissue,  but  are  developed  in  the  same  manner  as  the  primary 
capillaries  of  the  area  vasculosa,  by  the  aggregation  of  vasifactive 
cells  to  form  cords,  and  the  subsequent  hollowing  out  of  these. 
The  Formation  of  the  Blood. — The  haemoblasts,  which  are 
the  first  formed  blood-corpuscles  are  all  nucleated  and  destitute  or 
nearly  so  of  haemoglobin.  They  have  been  held  by  some  observers 
to  be  the  only  source  of  the  various  forms  of  corpuscles  that  are 
found  in  the  adult  vessels,  while  others  maintain  that  they  give 
rise  only  to  the  red  corpuscles,  the  leukocytes  arising  in  tissues 
external  to  the  blood-vessels  and  only  secondarily  making  their 
way  into  them.  According  to  this  latter  view  the  red  and  white 
corpuscles  have  a  different  origin  and  remain  distinct  throughout 
life. 

However  this  may  be,  it  is  certain  that  the  haemoblasts  and  the 
erythrocytes  that  are  formed  from  them  increase  by  division  in  the 
interior  of  the  embryo,  and  that  there  are  certain  portions  of  the 
body  in  which  these  divisions  take  place  most  abundantly,  partly, 
perhaps,  on  account  of  the  more  favorable  conditions  of  nutrition 
which  they  present  and  partly  because  they  are  regions  where  the 
circulation  is  sluggish  and  permits  the  accumulation  of  erythro- 
cytes. These  regions  constitute  what  have  been  termed  the 
hamatopoietic  organs,  and  are  especially  noticeable  in  the  later 
stages  of  fetal  life,  diminishing  in  number  and  variety  about  the 
time  of  birth.  It  must  be  remembered,  however,  that  the  life 
of  individual  corpuscles  is  comparatively  short,  their  death  and 
disintegration  taking  place  continually  during  the  entire  life  of 
the  individual,  so  that  there  is  a  necessity  for  the  formation  of  new 
corpuscles  and  for  the  existence  of  haematopoietic  organs  at  all 
stages  of  life. 

In  the  fetus  haemoblasts  in  process  of  division  may  be  found 
in  the  general  circulation  and  even  in  the  heart  itself,  but  they 
are  much  more  plentiful  in  places  where  the  blood-pressure  is 
diminished,  as,  for  instance,  in  the  larger  capillaries  of  the  lower 
limbs  and  in  the  capillaries  of  all  the  visceral  organs  and  of  the 


THE   FORMATION    OF   THE   BLOOD 


227 


subcutaneous  tissues.  Certain  organs,  however,  such  as  the  liver, 
the  spleen,  and  the  bone-marrow,  present  especially  favorable 
conditions  for  the  multiplication  of  the  blood-cells,  and  in  these- 
not  only  are  the  capillaries  enlarged,  so  as  to  afford  resting-places 
for  the  corpuscles,  but  gaps  appear  in  the  walls  of  the  vessels 
through  which  the  blood-elements  may  pass  and  so  come  into 
intimate  relations  with  the  actual  tissues  of  the  organs  (Fig.  154). 
After  birth  the  haematopoietic  function  of  the  Uver  ceases  and  that 
of  the  spleen  becomes  limited  to  the  formation  of  white  corpuscles^ 
though  the  complete  function  ^_  ^_^ 

may  be  re-established  in  cases  ^ 

of  extreme  anaemia.  The  bone 
marrow,  however,  retains  the 
function  completely,  being 
throughout  life  the  seat  of  for- 
mation of  both  red  and  white 
corpuscles,  the  lymphatic 
nodes  and  foUicles,  as  well  as 
the  spleen,  assisting  in  the  for- 
mation of  the  latter  elements. 

The  haemoblasts  early  be- 
come converted  into  nucleated 
red  corpuscles  or  erythrocytes 
by  the  development  of  haemo- 
globin in  their  cytoplasm,  their 
nuclei  at  the  same  time  becom- 
ing granular.  Up  to  a  stage 
at  which  the  embryo  has  a  length  of  about  112  mm.  these 
are  the  only  form  of  red  corpuscle  in  the  circulation,  but 
at  this  time  (Minot)  a  new  form,  characterized  by  its  smaller 
size  and  more  deeply  staining  nucleus,  makes  its  appearance. 
These  erythrocytes  have  been  termed  normoblasts  (Ehrhch), 
although  they  are  merely  transition  stages  leading  to  the  forma- 
tion of  erythroplastids  by  the  extrusion  of  their  nuclei  (Fig.  135). 
The  cast-off  nuclei  undergo  degeneration  and  phagocytic  absorp- 
tion by  the  leukcocytes,  and  the  masses  of  cytbplasm  pass  into 


Fig.  134. — Section  of  a  Portion  of  thb 
Liver  of  a  Rabbit  Embryo  of  5  mm. 

e.  Erythrocytes  in  the  liver  substance 
and  in  a  capillary;  h,  hepatic  cells. — {van 
der  Stricht.) 


228 


THE  FORMATION  OF  THE  BLOOD 


the  circulation,  becoming  more  and  more  numerous  as  develop- 
ment proceeds,  until  finally  they  are  the  typical  haemoglobin- 
containing  elements  in  the  blood  and  form  what  are  properly 
termed  the  red  blood-corpuscles. 

It  has  already  (p.  226)  been 
pointed  out  that  discrepant  views 
prevail  as  to  the  origin  of  the 
white  blood-corpuscles .  Indeed, 
three  distinct  modes  of  origin  have 
been  assigned  to  them.  According 
to  one  view  they  have  a  common 
origin  with  the  erythrocytes  from  the  haemoblasts  (Weidenreich), 
according  to  another  they  are  formed  from  mesenchyme  cells  out- 
side the  cavities  of  the  blood-vessels  (Maximow),  while  according 
to  a  third  view  the  first  formed  leukocytes  take  their  origin  from 
the  endodermal  epitheHal  cells  of  the  thymus  gland  (Prenant). 


9  &€)(§) 


Fig.  135.  —  Stages  in  the 
Transformation  of  an  Erythro- 
cyte  INTO    AN  ErYTHROPLASTID. 

(van  der  Stricht.) 


Fig.  136. — Figures  of  the  Different  Forms  of  White  Corpuscles  occurring 

IN  Human  Blood. 
■     a,  Lymphocytes;  h,  finely  granular  (neutrophile)  leukocyte;  c,  coarsely  granu- 
lar (eosinophile)  leukocyte;  d,  polymorphonuclear  (basophile)  leukocyte. — (Weiden- 
reich.) 


But  whatever  may  be  their  origin,  in  later  stages  the  leukocytes 
multiply  by  mitosis  and  there  is  a  tendency  for  the  dividing  cells  to 
collect  in  the  lymphoid  tissues,  such  as  the  lymph  nodes,  tonsils, 


THE    FORMATION    OF    THE   BLOOD 


229 


etc.,  to  form  more  or  less  definite  groups  which  have  been  termed 
germ-centers  (Flemming).  The  new  cells  when  they  first  pass 
into  the  circulation  have  a  relatively  large  nucleus  surrounded  by  aT 
small  amount  of  cytoplasm  without  granules  and,  since  they  re- 
semble the  cells  found  in  the  lymphatic  vessels,  are  termed 
lymphocytes  (Fig.  136,  a).  In  the  circulation,  however,  other 
forms  of  leukocytes  also  occur,  which  are  beheved  to  have  their 
origin  from  cells  with  much  larger  nuclei  and  more  abundant 
cytoplasm,  which  occur  throughout  life  in  the  bone-marrow  and 


Fig.  137. — Megacaryocyte   from    a    Kitten,    which    has   Extended   Two 

PSEUDOPODIAL  PROCESSES  THROUGH  THE  WaLL  OF  BlOOD-VESSEL  AND  IS  BUDDING 

OFF  Blood-platelets. 

hp,  Blotfd-platelets;  V,  blood-vessel. — {J.  H.  Wright.) 

have  been  termed  myelocytes.  Cells  of  a  similar  type,  free  in  the 
circulation,  constitute  what  are  termed  the  finejy  granular  leuko- 
cytes {neutrophile  cells  of  Ehrlich)  (Fig.  135,  b),  but  whether  these 
and  the  myelocytes  are  derived  from  lymphocytes  or  have  an 
independent  origin  is  as  yet  undetermined.  Less  abundant  are 
the  coarsely  granular  leukocytes  (eosinophile  cells  of  Ehrlich)  Fig. 
136,  c),  characterized  by  the  coarseness  and  staining  reactions  of 
their  cytoplasmic  granules  and  by  their  reniform  or  constricted 
nucleus.  They  are  probably  derivatives  of  the  finely  granular 
type  and  it  has  been  maintained  by  Weidenreich  that  their 
granules  have  been  acquired  by  the  phagocytosis  of  degenerated 


230 


THE   FORMATION    OF   THE   HEART 


erythrocytes.  Finally,  a  third  type  is  formed  by  the  poly- 
morphonuclear or  polynudear  leukocytes  (basophile  cells  of  Ehrlich) 
(Fig.  136,  d)j  which  are  to  be  regarded  as  leukocytes  in  the  process 
of  degeneration  and  are  characterized  by  their  irregularly  lobed 
or  fragmented  nuclei,  as  well  as  by  their  staining  peculiarities. 

In  the  fetal  haematopoietic  organs  and  in  the  bone-marrow  of 
the  adult  large,  so-called  giant-cells  are  found,  which,  although 
they  do  not  enter  into  the  general  circulation,  are  yet  associated 
with  the  development  of  the  blood-corpuscles.  These  giant-cells 
as  they  occur  in  the  bone-marrow  are  of  two  kinds  which  seem 
to  be  quite  distinct,  although  both  are  probably  formed  from 
leukocytes.  In  one  kind  the  cytoplasm  contains  several  nuclei, 
whence  they  have  been  termed  polycaryocyteSj  and  they  seem  to 
be  the  cells  which  have  already  been  mentioned  as  osteoclasts 
(p.  160).  In  the  other  kind  (Fig.  137)  the  nucleus  is  single,  but 
it  is  large  and  irregular  in  shape,  frequently  appearing  as  if  it  were 
producing  buds.  These  megacaryocytes  appear  to  be  phagocytic 
cells,  having  as  their  function  the  destruction  of  degenerated 
corpuscles  and  of  the  nuclei  of  the  erythrocytes. 

The  blood-platelets  have  recently  been  shown  by  Wright  to  be 
formed  from  the  cytoplasm  of  the  megacaryocytes,  by  the  constric- 
tion and  separation  of  portions  of  the  slender  processes  to  which 
they  give  rise  in  their  amoeboid  movements  (Fig.  137).  They 
have  also  been  described  as  forming  in  a  similar  manner  from 
leukocytes  and  even  from  the  endothehal  cells  of  the  blood  vessels 
(Jordan). 

The  Formation  of  the  Heart. — The  heart  makes  its  appearance 
while  the  embryo  is  still  spread  out  upon  the  surface  of  the  yolk 
sac,  and  arises  as  two  separate  portions  which  only  later  come  into 
contact  in  the  median  Une.  On  each  side  of  the  body  near  the 
margins  of  the  embryonic  area  a  fold  of  the  splanchnopleure 
appears,  projecting  into  the  coelomic  cavity,  and  within  this  fold  is 
a  thin-walled  sac,  probably  representing  an  enlargement  of  the 
primitive  angioblastic  network  (Fig.  138,  A).  Each  fold  will 
produce  a  portion  of  the  muscular  walls  (myocardium)  of  the  heart 
and  each  sac  part  of  its  endothehum   (endocardium).     As   the 


THE    FORMATION    OF   THE    HEART 


231 


constriction  of  the  embryo  from  the  yolk-sac  proceeds,  the  two 
folds  are  gradually  brought  nearer  together  (Fig.  13^,  B),  until 
they  meet  in  the  mid-ventral  Hne,  when  the  myocardial  folds  and 


en 


Fig.  138. — Diagrams  Illustrating  the  Formation  of  the  Heart  in  the  Guinea- 
pig. 
The  mesoderm  is  represented  in  black  and  the  endocardium  by  a  broken  line,     am. 
Amnion;  en,  endoderm;  h,  heart;  i,  digestive  tract. — (After  Strahl  and  Carius.) 

endocardial  sacs  fuse  together  (Fig.  13^,  C)  to  form  a  cylindrical 
heart  lying  in  the  mid-ventral  line  of  the  body,  in  front  of  the 
anterior  surface  of  the  yolk-sac  and  in  what  will  later  be  the 


232 


THE  FORMATION  OF  THE  HEART 


cervical  region  of  the  body.  At  an  early  stage  the  various  veins 
which  have  already  been  formed,  the  vitellines,  umbilicals,  jugu- 
lars and  cardinals,  unite  together  to  open  into  a  sac-like  structure, 
the  sinus  venosus,  and  this  opens  into  the  posterior  end  of  the  heart 
cylinder.  The  anterior  end  of  the  cylinder  tapers  off  to  form  the 
aortic  bulb,  which  is  continued  forward  on  the  ventral  surface  of 
the  pharyngeal  region  and  carries  the  blood  away  from  the 
heart.  The  blood  accordingly  opens  into  the  posterior  end  of 
the  heart  tube  and  flows  from  its  anterior  end. 


Fig.  139. — Heart  of  Embryo  of  2.15 
MM.,  FROM  A  Reconstruction. 
a.  Atrium;  ab,  aortic  bulb;  d,  dia- 
phragm; dc,  ductus  Cuvieri;  /,  liver; 
V,  ventricle;  vj,  jugular  vein;  vu,  um- 
bilical vein. — (His.) 


Fig.   140. — Heart  of  Embryo  of 

4.2      MM.,      SEEN     from      THE      DORSAL 

Surface. 

DC, Ductus  Cuvieri;  lA,  left  atrium; 
rA,  right  atrium;  vj,  jugular  vein;  VI, 
left  ventricle;  vu,  umbilical  vein. — 
(His.) 


The  simple  cylindrical  form  soon  changes,  however,  the  heart 
tube  in  embryos  of  2.15  mm.  in  length  having  become  bent  upon 
itself  into  a  somewhat  S-shaped  curve  (Fig.  139).  Dorsally  and 
to  the  left  is  the  end  into  which  the  sinus  venosus  opens,  and  from 
this  the  heart  tube  ascends  somewhat  and  then  bends  so  as  to  pass 
at  first  ventrally  and  then  caudally  and  to  the  right,  where  it 
again  bends  at  first  dorsally  and  then  anteriorly  to  pass  over  into 


THE    FORMATION    OF   THE    HEART 


233 


the  aortic  bulb.  The  portion  of  the  curve  which  lies  dorsally 
and  to  the  left  is  destined  to  give  rise  to  both  atria,  the  portion 
which  passes  from  right  to  left  represents  the  future  left  ventricle7 
while  the  succeeding  portion  represents  the  right  ventricle.  In 
later  stages  (Fig.  140)  the  left  ventricular  portion  drops  down- 
ward in  front  of  the  atrial  portion,  assuming  a  more  horizontal 
position,  while  the  portion  which  represents  the  right  ventricle 
is  drawn  forward  so  as  to  lie  in  the  same  plane  as  the  left. 

At  the  same  time  two  small  out-pouchings  develop  from  the 
atrial  part  of  the  heart  and  form  the  first  indications  of  the  two 


^n,fJ^^ 


atria.  As  development  pro- 
gresses, these  increase  in  size  to 
form  large  pouches  opening  into 
a  common  atrial  canal  (Fig.  141) 
which  is  directly  continuous  with 
the  left  ventricle,  and  as  the 
enlargement  of  the  pouches  con- 
tinues their  openings  into  the 
canal  enlarge,  until  finally  the 
pouches  become  continuous  with 
one  another,  forming  a  single 
large  sac,  and  the  atrial  canal 
becomes  reduced  to  a  short  tube 
which  is  slightly  invaginated  into 
the  ventricle  (Fig.  142). 

In  the  meantime  the  sinus  venosus,  which  was  originally  an 
oval  sac  and  opened  into  the  atrial  canal,  has  elongated  trans- 
versely until  it  has  assumed  the  form  of  a  crescent  whose  convexity 
is  in  contract  with  the  walls  of  the  atria,  and  its  opening  into  the 
heart  has  verged  toward  the  right,  until  it  is  situated  entirely 
within  the  area  of  the  right  atrium.  As  the  enlargement  of  the 
atria  continues,  the  right  horn  and  median  portion  of  the  crescent 
are  gradually  taken  up  into  their  walls,  so  that  the  various  veins 
which  originally  opened  into  the  sinus  now  open  directly  into  the 
right  atrium  by  a  single  opening  (Fig.  143)  guarded  on  either  side 
by  a  projecting  fold,  these  folds  being  continued  upon  the  roof 


Fig.  141. — Heart  of  Embryo  of  5 
MM.,  Seen  from  in  Front  and  slightly 
from  Above. — '{His.) 


234 


THE    FORMATION    OF    THE    HEART 


of  the  atrium  as  a  muscular  ridge,  known  as  the  septum  spurium 
(Fig.  42,  sp).  The  left  horn  of  the  crescent  is  not  taken  up  into 
the  atrial  wall,  but  remains  upon  its  posterior  surface  as  an  elon- 
gated sac,  forming  the  coronary  sinus. 

The  division  of  the  now  practically  single  atrial  cavity  into  the 
permanent  right  and  left  atria  begins  with  the  formation  of  a 
falciform  ridge  running  dorso-ventrally  across  the  roof  of  the 
cavity.     This  is  the  atrial  septum  or  septum  primum  (Fig.  142 


Fig.   142. — Inner  Surface  of  the  Heart  of  an  Embryo  of  10  mm. 

al,  Atrio-venticular  thickening;  sp,  septum  spurium;  ss,  septum  prim  am;  sv,  septum 

ventriculi;  ve.  Eustachian  valve. — {His.) 

ss),  and  it  rapidly  increases  in  size  and  thickens  upon  its  free 
margin,  which  reaches  almost  to  the  upper  border  of  the  short 
atrial  canal  (Fig.  144).  The  continuity  of  the  two  atria  is  thus 
almost  dissolved,  but  is  soon  re-established  by  the  formation  in 
the  dorsal  part  of  the  septum  of  an  opening  which  soon  reaches  a 
considerable  size  and  is  known  as  the  foramen  ovale  (Fig.  14^,  fo). 
Close  to  the  atrial  septum,  and  parallel  with  it,  a  second  ridge  ap- 
pears in  the  roof  and  ventral  wall  of  the  right  atrium.  This 
septum  secundum  (Fig.  143,  5^)  is  of  relatively  slight  development 


THE   FORMATION    OF   THE   HEART 


235 


Sr  S2 


in  the  human  embryo,  and  its  free  edge,   arching  around  the 
ventral  edge  and  floor  of  the  foramen  ovale,  becomes  continuous 
with  the  left  Kp  of  the  fold  which  guards  the  opening  of  the  sinus— 
venosus  and  with  this  forms  the  annulus  of  Vieussens  of  the 
adult  heart. 

When  the  absorption  of  the  sinus  venosus  into  the  wall  of  the 
right  atrium  has  proceeded  so  far  that  the  veins  communicate 
directly  with  the  atrium,  the  vena  cava  superior  opens  into  it  at 
the  upper  part  of  the  dorsal  wall,  the 
vena  cava  inferior  more  laterally,  and 
below  this  ^fe  the  smaller  opening  of  the 
coronary  sinus.  The  upper  portion  of 
the  right  lip  of  the  fold  which  originally 
surrounded  the  opening  of  the  sinus 
venosus,  together  with  the  septum 
spurium,  gradually  disappears;  the  lower 
portion  persists,  however,  and  forms  (i) 
the  Eustachian  valve  (Fig.  143,  Ve), 
guarding  the  opening  of  the  inferior  cava 
and  directing  the  blood  entering  by  it 
toward  the  foramen  ovale,  and  (2)  the 
Thebesian  valve,  which  guards  the  open- 
ing of  the  coronary  sinus.  At  first  no 
veins  communicate  with  the  left  atrium, 
but  on  the  development  of  the  lungs 
and  the  establishment  of  their  vessels,  the 
pulmonary  veins  make  connection  with  it.  Two  veins  arise  from 
each  lung,  and  asthey  pass  toward  the  heart  they  unite  in  pairs,  the 
two  vessels  so  formed  again  uniting  to  form  a  single  short  trunk 
which  opens  into  the  upper  part  of  the  atrium  (Fig.  144,  Vep). 
As  is  the  case  with  the  right  atrium  and  the  sinus  venosus,  the 
expansion  of  the  left  atrium  brings  about  the  absorption  of  the 
short  single  trunk  into  its  walls,  and,  the  expansion  continuing, 
the  two  vessels  are  also  absorbed,  so  that  eventaully  the  four  pri- 
mary veins  open  independently  into  the  atrium. 

While  the  atrial  septa  have  been  developing  there  has  appeared 


Fig.  143. — Heart  of  Em- 
bryo OF  10.2  CM.  FROM  WHICH 

Half  of  the  Right  Auricle 
HAS  BEEN  Removed] 

/o,  Foramen  ovale;  pa, 
pulmonary  artery;  Si  septum 
primum;  Si,  septum  sec- 
undum; Sa,  systemic  aorta; 
V,  right  ventricle;  vci  and  vcs, 
inferior  and  superior  venae 
cavae;  Ve,  Eustachian  valve. 


236 


THE    FORMATION    OF   THE    HEART 


on  the  dorsal  wall  of  the  atrial  canal  a  tubercle-like  thickening  of 
the  endocardium,  and  a  similar  thickening  also  forms  on  the 
ventral  wall.  These  endocardial  cushions  increase  in  size  and 
finally  unite  together  by  their  tips,  forming  a  complete  parti- 
tion, dividing  the  atrial  canal  into  a  right  and  left  half  (Fig.  144). 


SM 


En.s 


Bw^ 


Fig.  144. — Section  through  a  Reconstruction  of  the  Heart  of  a  Rabbit 

Embryo  of  10. i  mm. 
Ad  and  Adi,  Right  and  As,  left  atrium;  Bwi  and  Bw2,  lower  ends  of  the  ridges 
which  divide  the  aortic  bulb;  En,  endocardial  cushion;  En.r  and  En.s,  thickenings 
of  the  cushion;  la,  interatrial  and  Iv,  interventricular  communication;  Si,  septum 
primum;  Sd,  right  and  55,  left  horn  of  the  sinus  venosus;  S.iv,  ventricular  septum; 
SM,  opening  of  the  sinus  venosus  into  the  atrium;  Vd,  right  and  Vs,  left  ventricle; 
Vej,  jugular  vein;  Vep,  pulmonary  vein;  Vvd  and  Vvs,  right  and  left  limbs  of  the 
valve  guarding  the  openings  of  the  sinus  venosus. — (Born.) 

With  the  upper  edge  of  this  partition  the  thickened  lower  edge  of 
the  atrial  septum  unites,  so  that  the  separation  of  the  atria  would 
be  complete  were  it  not  for  the  foramen  ovale. 

While  these  changes  have  been  taking  place  in  the  atrial  por- 


THE   FORMATION    OF    THE   HEART  237 

tion  of  the  heart,  the  separation  of  the  right  and  left  ventricles  has 
also  been  progressing,  and  in  this  two  distinct  septa  take  part. 
From  the  floor  of  the  ventricular  cavity  along  the  line  of  junction" 
of  the  right  and  left  portions  a  ridge,  composed  largely  of  muscular 
tissue,  arises  (Figs.  142  and  144),  and,  growing  more  rapidly  in  its 
dorsal  than  its  ventral  portion,  it  comes  into  contact  and  fuses 
with  the  dorsal  part  of  the  partition  of  the  atrial  canal.  Ventrally, 
however,  the  ridge,  known  as  the  venrHcular  septum,  fails  to 
reach  the  ventral  part  of  the  partition,  so  that  an  oval  foramen, 
situated  just  below  the  point  where  the  aortic  bulb  arises,  still 
remains  between  the  two  ventricles.  This  opening  is  finally 
closed  by  what  is  termed  the  aortic  septum.  This  makes  its 
appearance  in  the  aortic  bulb  just  at  the  point  where  the  first 
lateral  branches  which  give  origin  to  the  pulmonary  arteries  (see 
p.  245)  arise,  and  is  formed  by  the  fusion  of  the  free  edges  of  two 
endocardial  ridges  which  develop  on  opposite  sides  of  the  bulb. 
From  its  point  of  origin  it  gradually  extends  down  the  bulb  until 
it  reaches  the  ventricle,  where  it  fuses  with  the  free  edge  of  the 
ventricular  septum  and  so  completes  the  separation  of  the  two 
ventricles  (Fig.  145).  The  bulb  now  consists  of  two  vessels  lying 
side  by  side,  and  owing  to  the  position  of  the  partition .  at  its 
anterior  end,  one  of  these  vessels,  that  which  opens  into  the  right 
ventricle,  is  continuous  with  the  pulmonary  arteries,  while  the 
other,  which  opens  into  the  left  ventricle,  is  continuous  with  the 
rest  of  the  vessels  which  arise  from  the  forward  continuation  of 
the  bulb.  As  soon  as  the  development  of  the  partition  is  com- 
pleted, two  grooves,  corresponding  in  position  to  the  lines  of  at- 
tachment of  the  partition  on  the  inside  of  the  bulb,  make  their 
appearance  on  the  outside  and  gradually  deepen  until  they  finally 
meet  and  divide  the  bulb  into  two  separate  vessels,  one  of  which 
is  the  pulmonary  aorta  and  the  other  the  systemic  aorta. 

In  the  early  stages  of  the  heart's  development  the  muscle 
bundles  which  compose  the  wall  of  the  ventricle  are  very  bosely 
arranged,  so  that  the  ventricle  is  a  somewhat  spongy  mass  of 
muscular  tissue  with  a  relatively  small  cavity.  As  development 
proceeds  the  bundles  nearest  the  outer  surface  come  closer  to- 


238 


THE   FORMATION    OF    THE   HEART 


gether  and  form  a  compact  layer,  those  on  the  inner  surface,  how- 
ever, retaining  their  loose  arrangement  for  a  longer  time  (Fig. 


S.Tir 


App     T 


:Fa?:d 


Fay.s 


-  - — Sw 


-Vs 


S.ivr 


Fig.  145. — Diagrams  of  Sections  through  the  Heart  of  Embryo  Rabbits 
TO  Show  the  Mode  of  Division  of  the  Ventricles  and  of  the  Atrio-ventricu- 
lar  Orifice. 

Ao,  Aorta;  Ar.p,  pulmonary  artery;  B,  aortic  bulb;  Bw2  and  *,  one  of  the  ridges 
which  divide  the  bulb;  Eo,  and  Eu,  upper  and  lower  thickenings  of  the  margins  of 
the  atrio-ventricular  orifice;  F.av.c,  the  original  atrio-ventricular  orifice;  F.av.d  and 
F.av.s,  right  and  left  atrio-ventricular  orifices;  Oi,  interventricular  communication; 
5.  iv,  ventricular  septum;  Vd  and  Vs,  right  and  left  ventricles. — {Born.) 

144).  The  lower  edge  of  the  atrial  canal  becomes  prolonged  on 
the  left  side  into  one,  and  on  the  right  side  into  two,  flaps  which 
project '  downward  into  the  ventricular  cavity,  and  an  additional 


THE    FORMATION   OF   THE   HEART 


239 


flap  arises  on  each  side  from  the  lower  edge  of  the  partition  of  the 
atrial  canal,  so  that  three  flaps  occur  in  the  right  atrio-ven- 
tricular  opening  and  two  in  the  left.  To  the  under  surfaces  of 
these  flaps  the  loosely  arranged  muscular  trabeculae  of  the  ventricle 
are  attached,  and  muscular  tissue  also  occurs  in  the  flaps.  This 
condition  is  transitory,  however;  the  muscular  tissue  of  the  flaps 
degenerates  to  form  a  dense  layer  of  connective  tissue,  and  at  the 
same  time  the  muscular  trabeculae  undergo  a  condensation.  Some 
of  them  separate  from  the  flaps,  which  represent  the  atrio-ventricu- 
lar  valves,  and  form  muscle  bundles  which  may  fuse  throughout 
their  entire  length  with  the  more  compact  portions  of  the  ventricu- 


PiG.  146. — Diagrams  showing  the  Development  of  the  Auriculo-ventricular 

Valves. 

b,  Muscular  trabeculae;   cht,  chordae  tendineae;  mk  and  mk^,  valve;  pm,  musculus 

papillaris;  tc,  trabeculae  carneae;  v,  ventricle. — (From  Hertwig,  after  Gegenbaur.) 

lar  walls,  or  else  may  be  attached  only  by  their  ends,  forming 
loops;  these  two  varieties  of  muscle  bundles  constitute  the  tra- 
heculcB  carnecB  of  the  adult  heart.  Other  bundles  may  retain  a 
transverse  direction,  passing  across  the  ventricular  cavity  and 
forming  the  so-called  moderator  hands;  while  others,  again,  re- 
taining their  attachment  to  the  valves,  condense  only  at  their 
lower  ends  to  form  the  musculi  papillares,  their  upper  portions 
undergoing  conversion  into  strong  though  slender  fibrous  cords, 
the  chordm  tendinece  (Fig.  146). 

The  endocardial  lining  of  the  ventricles  is  at  first  a  simple  sac 
separated  by  a  distinct  interval  from  the  myocardium,  but  when 
the  condensation  of  the  muscle  trabeculae  occurs  the  endocardium 
applies  itself  closely  to  the  irregular  surface  so  formed,  dipping 


240 


THE  PORMATION  OF  THE  HEART 


into  all  the  crevices  between  the  trabeculae  carneae  and  wrapping 
itself  around  the  musculi  papillares  and  chordae  tendinae  so  as  to 
form  a  complete  lining  of  the  inner  surface  of  the  myocardium. 
In  early  stages  the  myocardial  tissue  of  the  atria  is  continuous 
with  that  of  the  ventricles  throughout  the  entire  circumference  of 
the  wall  of  the  atrial  canal,  but  later  this  wall  becomes  converted 
into  connective  tissue  and  the  continuity  is  interrupted,  except  in 
the  region  behind  the  posterior  endocardial  cushion.  Here  a 
band  of  the  original  tissue  persists  and  eventually  forms  the 
atrichventricular  bundle. 

The  aortic  and  pulmonary  semilunar 
valves  make  their  appearance,  before  the 
aortic  bulb  undergoes  its  longitudinal  split- 
ting, as  four  tubercle-Uke  thickenings  of  con- 
nective tissue  situated  on  the  inner  wall  of 
the  bulb  just  where  it  arises  from  the  ventricle. 
When  the  division  of  the  bulb  occurs,  two 
of  the  thickenings,  situated  on  opposite  sides, 
are  divided,  so  that  both  the  pulmonary 
and  systemic  aortae  recfcive  three  thickenings 
(Fig.  147).  Later  the  thickenings  become  hollowed  out  on  the 
surfaces  directed  away  from  the  ventricles  and  are  so  converted 
into  the  pouch-like  valves  of  the  adult. 

Changes  in  the  Heart  after  Birth. — The  heart  when  first  formed 
lies  far  forward  in  the  neck  region  of  the  embryo,  between  the 
head  and  the  anterior  surface  of  the  yolk-sac,  and  from  this  posi- 
tion it  gradually  recedes  until  it  reaches  its  final  position  in  the 
thorax.  And  not  only  does  it  thus  change  its  relative  position,  but 
the  direction  of  its  axes  also  changes.  For  at  an  early  stage  the 
ventricles  lie  directly  in  front  of  {i.e.,  ventrad  to)  the  atria  and  not 
below 'them  as  in  the  adult  heart,  and  this  primitive  condition  is 
retained  until  the  diaphragm  has  reached  its  final  position  (see 

p.  325). 

In  addition  to  these  changes  in  position,  which  are  antenatal, 
important  changes  also  occur  in  the  atrial  septum  after  birth. 
Throughout   the  entire  period  of  fetal  life  the  foramen  ovale 


Fig.  147.  —  Dia- 
grams Illustrating 
THE  Formation  of  the 
Semilunar  Valves. — 
(fiegenhaur.) 


DEVELOPMENT    OF    THE   ARTERIAL   SYSTEM  241 

persists,  permitting  the  blood  returning  from  the  placenta  and 
entering  the  right  atrium  to  pass  directly  across  to  the  left  atrium, 
thence  to  the  left  ventricle,  and  so  out  to  the  body  through  the 
systemic  aorta  (see  p.  268).  At  birth  the  lungs  begin  to  function 
and  the  placental  circulation  is  cut  off,  so  that  the  right  atrium 
receives  only  venous  blood  and  the  left  only  arterial;  a  persistence 
of  the  foramen  ovale  beyond  this  period  would  be  injurious,  since 
it  would  permit  of  a  mixtue  of  the  arterial  and  venous  bloods,  and, 
consequently,  it  closes  completely  soon  after  birth.  The  closure  is 
made  possible  by  the  fact  that  during  the  growth  of  the  heart  in 
size  the  portion  of  the  atrial  septum  which  is  between  the  edge  of 
the  foramen  ovale  and  the  dorsal  wall  of  the  atrium  increases  in 
width,  so  that  the  foramen  is  carried  further  and  further  away 
from  the  dorsal  wall  of  the  atrium  and  comes  to  be  almost  com- 
pletely overlapped  by  the  annulus  of  Vieussens  (Fig.  143).  This 
process  continuing,  the  dorsal  portion  of  the  atrial  septum  finally 
overlaps  the  free  edge  of  the  annulus,  and  after  birth  the  fusion  of 
the  overlapping  surfaces  takes  place  and  the  foramen  is  com- 
pletely closed. 

In  a  large  percentage  (25  to  30  per  cent.)  of  individuals  the  fusion  of 
the  surfaces  of  the  septum  and  annulus  is  not  complete,  so  that  a  slit- 
like opening  persists  between  the  two  atria.  This,  however,  does  not 
'allow  of  any  mingling  of  the  blood  in  the  two  cavities,  since  when  the 
atria  contract  the  pressure  of  the  blood  on  both  sides  will  force  the 
overlapping  folds  together  and  so  practically  close  the  opening.  Occa- 
sionally the  growth  of  the  dorsal  portion  of  the  septum  is  imperfect  or 
is  inhibited,  in  which  case  closure  of  the  foramen  ovale  is  impossible. 

The  Development  of  the  Arterial  System.— It  has  been  seen 
(p.  222)  that  the  formation  of  the  blood-vessels  begins  in  the  extra- 
embryonic splanchnic  mesoderm  surrounding  the  yolk-sac  and  ex- 
tends thence  toward  the  embryo.  Furthermore,  it  has  been  seen 
that  the  vessels  appear  as  capillary  networks  from  which  definite 
stems  are  later  elaborated.  This  seems  also  to  be  the  method  of 
formation  of  the  vessels  developed  within  the  body  of  the  embryo, 
the  arterial  and  venous  stems  being  .first  represented  by  a  number 
of  anastomosing  capillaries,  from  which,  by  the  enlargement  of 

16 


242 


DEVELOPMENT   OF   THE   ARTERIAL    SYSTEM 


some  and  the  disappearance  of  the  others,  the  definite  stems  are 
formed. 

The  earliest  known  embryo  that  shows  a  blood  circulation  is 
that  described  by  Eternod  (Fig.  44).  From  the  plexus  of  vessels 
on  the  yolk-sac  two  veins  arise  which  unite  with  two  other  veins 
returning  from  the  chorion  by  the  belly-stalk  and  passing  forward 
to  the  heart  as  the  two  umbihcal  veins  (Fig.  148,  Vu).  There  is 
as  yet  no  vitelline  vein,  the  chorionic  circulation  in  the  human 


Fig.  148. — Diagram  showing  the  Arrangement  of  the  Blood-vessels    in  an 

Embryo  1.3  mm.  in  Length. 
Au,  Umbilical  artery;  All,  allantois;  Ch,  chorionic  villus;  dAr  and  dAs,  right  and  left 
dorsal  aortae;  Vu,  umbilical  veins;  Ys,  yolk-sac. — (From  Kollmann  after  Eternod.) 

embryo  apparently  taking  precedence  over  the  vitelline.  From 
the  heart  a  short  arterial  stem  arises,  which  soon  divides  so  ?s  to 
form  three  branches*  passing  dorsally  on  either  side  of  the  phar- 
ynx.    The  branches  of  each  side  then  unite  to  form  a  paired  dorsal 

*  Evans  (Keibel-Mall,  Human  Embryology,  Vol.  n,  191 2)  considers  two  of  these 
branches  to  be  probably  plexus  formations  rather  than  definite  stems,  since  there 
is  evidence  to  indicate  that  only  one  such  stem  exists  at  such  an  early  stage  of 
development. 


DEVELOPMENT    OF   THE    ARTERIAL    SYSTEM 


243 


aorta  (dAr,  dAs)  which  extends  caudally  and  is  continued  into  the 
belly-stalk  and  so  to  the  chorion  as  the  umbiHcal  arteries  (Au). 
There  is  as  yet  no  sign  of  vitelline  arteries  passing  to  the  yolk-sac, 
again  an  indication  of  the  subservience  of  the  vitelline  to  the 
chorionic  circulation  in  the  human  embryo. 

In  later  stages  when  the 
branchial  arches  have  appeared 
the  dorsally  directed  arteries  are 
seen  to  he  in  these,  forming  what 
are  termed  the  branchial  arch 
vessels,  and  later  also  the  two 
dorsal  aortae  fuse  as  far  forward 
as  the  region  of  the  eighth 
cervical  segment  to  form  a  single 
trunk  from  which  segmental 
branches  arise. 

It  will  be  convenient  to  con- 
sider first  the  history  of  the  ves- 
sels which  pass  dorsally  in  the 
branchial  arches.  Altogether, 
six  of  these  vessels  are  devel- 
oped, the  fifth  rudimentary  and 
transitory,  and  when  fully  formed 
they  have  an  arrangement  which 
may  be  understood  from  the  dia- 
gram (Fig.  149).  This  arrange- 
ment represents  a  condition  which  is  permanent  in  the  lower  verte- 
brates. In  the  fishes  the  respiration  is  performed  by  means  of 
gills  developed  upon  the  branchial  arches,  and  the  heart  is  an 
organ  which  receives  venous  blood  from  the  body  and  pumps  it 
to  the  gills,  in  which  it  becomes  arterialized  and  is  then  collected 
into  the  dorsal  aortae,  which  distribute  it  to  the  body.  But  in 
terrestrial  animals ,' with  the  loss  of  gills  and  the  development  of  the 
lungs  as  respiratory  organs,  the  capillaries  of  the  gills  disappear 
and  the  afferent  and  efferent  branchial  vessels  become  continuous, 
the  condition  represented  in  the  diagram  resulting. 


Fig.  149. — Diagram  Illustrating 
THE  Primary  Arrangement  of  the 
Branchial  Arch  Vessels. 

■  a.  Aorta;  ab,  aortic  bulb;  ec,  external 
carotid;  ic,  internal  carotid;  sc,  sub- 
clavian; I-VI,  branchial  arch  vessels. 


244  DEVELOPMENT    OF    THE    ARTERIAL    SYSTEM 

But  this  condition  is  merely  temporary  in  the  mammalia  and 
numerous  changes  occur  in  the  arrangement  of  the  vessels  before 
the  adult  plan  is  realized.  The  first  change  is  a  disappearance  of 
the  vessels  of  the  first  arch,  the  ventral  stem  from  which  it  arose 
being  continued  forward  to  form  the  temporal  arteries,  giving  off 
near  the  point  where  the  branchial  vessel  originally  arose  a  branch 
which  represents  the  internal  maxillary  artery  in  part,  and  possibly 
also  a  second  branch  which  represents  the  external  maxillary  (His). 
A  little  later  the  second  branchial  vessel  also  degenerates  (Fig. 


Pig.   ISO. — Arterial  System  of  an  Embryo  of  io  mm. 
Ic,  Internal  carotid;  P,  pulmonary  artery;  Ve,  vertebral  artery;  ///  to  VI,  persistent 
branchial  vessels. — (His.) 

150),  a  branch  arising  from  the  ventral  trunk  near  its  former 
origin,  possibly  representing  the  future  lingual  artery  (His),  and 
then  the  portion  of  the  dorsal  trunk  which  intervenes  between  the 
third  and  fourth  branchial  vessels  vanishes,  so  that  the  dorsal 
trunk  anterior  to  the  third  branchial  arch  is  cut  off  from  its  con- 
nection with  the  dorsal  aorta  and  forms,  together  with  the  vessel  of 
the  third  arch,  the  internal  carotid,  while  the  ventral  trunk, 
anterior  to  the  point  of  origin  of  the  third  vessel,  becomes  the 
external  carotid,  and  the  portion  which  intervenes  between  the 
third  and  fourth  vessels  becomes  the  common  carotid  (Fig.  151). 
The  rudimentary  fifth  vessel,  like  the  first  and  second,  dis- 


DEVELOPMENT    OF   THE    ARTERIAL    SYSTEM 


245 


appears,  but  the  fourth  persists  to  form  the  aortic  arch,  there  being 
at  this  stage  of  development  two  complete  aortic  arches.  From 
the  sixth  vessel  a  branch  arises  which  passes  backward  to  unite 
with  a  network  of  vessels  which  extends  downwards  to  the  region 
of  the  lungs  and  is  formed 
in  cat  embryos  by  the  an- 
astomosis of  branches  from 
the  upper  six  segmental 
branches  of  the  dorsal 
aortae  (Huntington).  From 
this  network  the  pulmonary 
artery  eventually  differen- 
tiates and,  its  connections 
with  the  segmental  aortic 
branches  dissolving,  it  ap- 
pears to  be  a  direct  down- 
growth  from  the  sixth  arch. 
The  portion  of  the  right 
sixth  arch  that  intervenes 
between  the  point  of  origin 
of  the  pulmonary  artery  and 
the  right  aortic  arch  disap- 
pears, while  the  correspond- 
ing portion  of  the  left  side 
persists  until  after  birth, 
forming  the  ductus  arteriosus 
(ductus  Botalli)  (Fig.  151). 
When  the  longitudinal  di- 
vision   of    the    aortic    bulb 

occurs  (p.  237),  the  septum  is  so  arranged  as  to  place  the  sixth 
arch  in  communication  with  the  right  ventricle  and  the  remain- 
ing vessels  in  connection  with  the  left  ventricle,  the  only  direct 
communication  between  the  systemic  and  pulmonary  vessels 
being  by  way  of  the  ductus  arteriosus,  whose  significance  will  be 
explained  later  (p.  269). 

One  other  change  is  still  necessary  before  the  vessels  acquire 


Fig.  151. — Diagram  Illustrating  the 
Changes  in  the  Branchial  Arch  Vessels. 

a.  Aorta;  da,  ductus  arteriosus;  ec,  ex- 
ternal carotid;  ic,  internal  carotid;  pa, 
pulmonary  artery;  sc,  subclavian;  I-IV, 
aortic  arch  vessels. 


246 


DEVELOPMENT   OF   THE   ARTERIAL   SYSTEM 


the  arrangement  which  they  possess  during  fetal  life,  and  this 
consists  in  the  disappearance  of  the  lower  portion  of  the  right 
aortic  arch  (Fig.  151),  so  that  the  left  arch  alone  forms  the  con- 
nection between  the  heart  and  the  dorsal  aorta.     The  upper  part 

of  the  right  aortic  arch  persists 
to  form  the  proximal  part  of  the 
right  subclavian  artery,  the  por- 
tion of  the  ventral  trunk  which 
unites  the  arch  with  the  aortic 
bulb  becoming  the  innominate 
artery. 

From  the  entire  length  of  the 

thoracic  aorta,  and  in  the  embryo 

from   the   aortic  arches,  lateral 

-i[-    ;^\  1]    n.^      branches  arise  corresponding  to 

3C       M,  /^^  ^       ^^^^  segment  and  accompanying 

the  segmental  nerves.     The  first 
of  these  branches  arises  just  be- 
low the  point  of  union  of  the 
vessel  of  the  sixth  arch  with  the 
dorsal   trunk   and  accompanies 
the     hypoglossal     nerve     (Fig. 
152,  /f),  and  that  which  accom- 
panies   the    seventh    cervical 
nerve  arises  just  above  the  point 
of  union  of  the  two  aortic  arches 
(Fig.    152,  s),   and  extends  out 
into  the  Hmb  bud,  forming  the 
subclavian  artery.* 
Further  down  twelve  pairs  of  lateral  branches,  arising  from 
the  thoracic  portion  of  the  aorta,  represent  the  intercostal  arteries, 
and  still  lower  four  pairs  of  lumbar  arteries  are  formed,  the  fifth 
lumbars   being  represented  by  two  large  branches,  the  common 

*  It  must  be  remembered  that  the  right  subclavian  of  the  adult  is  more  than 
equivalent  to  the  left,  since  it  represents  the  fourth  branchial  vessel  -f-  a  portion  of 
the  dorsal  longitudinal  trunk  -1-  the  lateral  segmental  branch  (see  Fig.  144). 


iCo.     IM 


Fig.  152. — Diagram  showing  the 
Relations  of  the  Lateral  Branches 
TO  the  Aortic  Arches. 

EC,  External  carotid;  h,  lateral 
branch  accompanying  the  hypoglossal 
nerve;  /C,  internal  carotid;  ICo,  inter- 
costal; IM,  internal  mammary;  s,  sub- 
clavian; V,  vertebral;  /to  VIII,  lateral 
cervical  branches;  i,  2,  lateral  thoracic 
branches. 


DEVELOPMENT    OF    THE    ARTERIAL    SYSTEM 


247 


iliacs,  which  seem  from  their  size  to  be  the  continuations  of  the 
aorta  rather  than  branches  of  it.  The  true  continuation  of  the 
aorta  is,  however,  the  middle  sacral  artery,  which  represents  in  a" 
degenerated  form  the  caudal  prolongation  of  the  aorta  of  other 
mammals,  and,  like  this,  gives  off  lateral  branches  corresponding 
to  the  sacral  segments. 

In  addition  to  the  segmental  lateral  branches  arising  from  the 
aorta,  visceral  branches,  which  have  their  origin  rather  from  the 


Fig.  153. — Diagram  showing  the  Arrangement  of  the  Segmental  Branches 

ARISING    from    the    AoRTA, 

A.  Aorta;  B,  lateral  somatic  branch;  C,  lateral  visceral  branch;  D,  median  visceral 
branch;  E,  peritoneum. 

ventral  surface,  also  occur.  In  embryos  of  5  mm.  these  branches 
are  arranged  in  a  segmental  manner  in  threes,  a  median  unpaired 
vessel  passing  to  the  digestive  tract  and  a  pair  of  more  lateral 
branches  passing  to  the mesonephros  (see  p.  342), corresponding  to 
each  of  the  paired  branches  passing  to  the  body  wall  (Fig.  153). 
As  development  proceeds  the  great  majority  of  these  visceral 
branches  disappear,  certain  of  the  lateral  ones  persisting,  however, 
to  form  the  renal,  internal  spermatic,  and  hypogastric  arteries  of 
the    adult,    while    the  unpaired  branches   are  represented  only 


248 


DEVELOPMENT    OF   THE   ARTERIAL   SYSTEM 


by  the  coeliac  artery  and  the  superior  and  inferior  mesenteries. 
The  superior  mesenteric  artery  is  the  adult  representative  of  the 
vitelline  artery  of  the  embryo  and  arises  from  the  aorta  by  two, 
three  or  more  roots,  v\rhich  correspond  to  the  fifth,  fourth  and 
higher  thoracic  segments.  Later,  all  but  the  lowest  of  the  roots 
disappear  and  the  persisting  one  undergoes  a  downward  migra- 
tion in  accordance  with  the  recession  of  the  diaphragm  and  viscera 

(see  p.  325)  until  in  embryos 
of  1 7  mm.  it  lies  opposite  the 
first  lumbar  segment.  Sim- 
ilarly the  coeliac  and  inferior 
mesenteric  arteries,  which 
when  first  recognizable  in  em- 
bryos of  9  mm.  correspond 
with  the  fourth  and  twelfth 
thoracic  segments  respect- 
ively, also  undergo  a  second- 
ary downward  migration,  the 
coeHac  artery  in  embryos  of 
17  mm.  arising  opposite  the 
twelfth  thoracic  and  the  in- 
ferior mesenteric  opposite  the 
third  lumbar  segment. 

The  umbilical  arteries  of 
the  embryo  seem  at  first  to  be 
the  direct  continuations  of  the 
dorsal  aortae  (Fig.  148),  but  as 
development  proceeds  they  come  to  arise  from  the  aorta  opposite 
the  third  lumbar  segment,  where  they  are  in  line  with  the  lateral 
visceral  segmental  branches.  They  pass  ventral  to  the  Wolffian 
duct  (see  p.  341)  and  are  continued  out  along  with  the  allantois 
to  the  chorionic  villi.  Later  this  original  stem  is  joined,  not  far 
from  its  origin,  by  what  appears  to  be  the  lateral  somatic  branch 
of  the  fifth  lumbar  segment,  whereupon  the  proximal  part  of  the 
original^  umbilical  vessel  degenerates  and  the  umbilical  comes  to 
arise  from  the  somatic  branch  which  is  the  common  iliac  artery 


Fig.  154. — Diagram  Illustrating  the 
Development  of  the  Umbilical  Arteries. 

A,  Aorta;  CIl,  common  iliac;  Ell,  ex- 
ternal iliac;  G,  gluteal;  ///,  internal  iliac; 
IP,  internal  pudic;  IV,  inferior  vesical;  Sc, 
sciatic;  U,  umbilical;  U',  primary  proximal 
portion  of  the  umbilical ;  wd.  Wolffian  duct. 


DEVELOPMENT    OF   THE   ARTERIAL   SYSTEM  249 

of  adult  anatomy  (Fig.  154).  Hence  it  is  that  this  vessel  in  the 
adult  gives  origin  both  to  branches  such  as  the  external  iliac,  the 
gluteal,  the  sciatic  and  the  internal  pudendal,  which  are  distrib- 
uted to  the  body  walls  or  their  derivatives,  and  to  others,  such  as 
the  vesical,  inferior  haemorrhoidal  and  uterine,  which  are  dis- 
tributed to  the  pelvic  viscera.  At  birth  the  portions  of  the  um- 
bilical arteries  beyond  the  umbilicus  are  severed  when  the  umbiHcal 
cord  is  cut,  and  their  intraembryonic  portions,  which  have  been 
called  the  hypogastric  arteries,  quickly  undergo  a  reduction  in  size. 
Their  proximal  portions  remain  functional  as  the  superor  vesical 
arteries,  carrying  blood  to  the  urinary  bladder,  but  the  portions 
which  intervene  between  the  bladder  and  the  umbilicus  become 
reduced  to  solid  cords,  forming  the  obliterated  hypogastric  arteries 
of  adult  anatomy. 

In  its  general  plan,  accordingly,  the  arterial  system  may  be 
regarded  as  consisting  of  a  pair  of  longitudinal  vessels  which  fuse 
together  throughout  the  greater  portion  of  their  length  to  form 
the  dorsal  aorta,  from  which  there  arise  segmentally  arranged 
lateral  somatic  branches  and  ventral  and  lateral  visceral  branches. 
With  the  exception  of  the  aortic  trunks  (together  with  their  an- 
terior continuations,  the  internal  carotids)  and  the  external  caro- 
tids, no  longitudinal  arteries  exist  primarily.  In  the  adult, 
however,  several  longitudinal  vessels,  such  as  the  vertebrals, 
internal  mammary,  and  epigastric  arteries,  exist.  The  formation 
of  these  secondary  longitudinal  trunks  is  the  result  of  a  develop- 
ment between  adjacent  vessels  of  anastomoses,  which  become 
larger  and  more  important  blood-channels  than  the  original  vessels . 

At  an  early  stage  each  of  the  lateral  branches  of  the  dorsal  aorta 
gives  off  a  twig  which  passes  forward  to  anastomose  with  a  back- 
wardly  directed  twig  from  the  next  anterior  lateral  branch,  so  as  to 
form  a  longitudinal  chain  of  anastomoses  along  each  side  of  the 
neck.  In  the  earliest  stage  at  present  known  the  chain  starts  from 
the  lateral  branch  corresponding  to  the  first  cervical  (suboccipital) 
segment  and  extends  forward  into  the  skull  through  the  foramen 
magnum,  terminating  by  anastomosing  with  the  internal  carotid. 
To  this  original  chain  other  links  are  added  from  each  of  the  sue- 


250 


DEVELOPMENT    OF    THE    ARTERIAL   SYSTEM 


ceeding  cervical  lateral  branches  as  far  back  as  the  seventh  (Pigs. 
152  and  155).  But  in  the  meantime  the  recession  of  the  heart 
toward  the  thorax  has  begun,  with  the  result  that  the  common 
carotid  stems  are  elongated  and  the  aortic  arches  are  apparently 


Avcb. 


ISp.G. 


Avcv 


Pig.  155. — The  Development  of  the  Vertebral  Artery  in  a  Rabbit  Embryo 

OF  Twelve  Days. 
IIIA.B  to  VIA.B,  Branchial  arch  vessels;  Ap,  pulmonary  artery.     A.v.c.b  and 
A.v.cv,  cephalic  and  cervical  portions  of  the  vertebral  artery;  A.s,  subclavian;  C.d 
and  C.v  internal  and  external  carotid;  ISp.G,  spinal  ganglion. — (Hochstetter.) 

shortened  so  that  the  subclavian  arises  on  the  left  side  almost 
opposite  the  point  where  the  aorta  was  joined  by  the  sixth  bran- 
chial vessel.  As  this  apparent  shortening  proceeds,  the  various 
lateral  branches  which  give  rise  to  the  chain  of  anastomoses,  with 
the  exception  of  the  seventh,  disappear  in  their  proximal  por- 


DEVELOPMENT    OF   THE    ARTERIAL   SYSTEM  25 1 

tions  and  the  chain  becomes  an  independent  stem,  the  vertebral 
artery,  arising  from  the  seventh  lateral  branch,  which  is  the  sub- 
clavian. 

The  recession  of  the  heart  is  continued  until  it  lies  below  the 
level  of  the  upper  intercostal  arteries,  and  the  upper  two  of  these, 
together  with  the  last  cervical  branch  on  each  side,  lose  their  con- 
nection with  the  dorsal  aorta,  and,  sending  off  anteriorly  and  pos- 
teriorly anastomosing  twigs,  develop  a  short  longitudinal  stem, 
the  costo-cervical  trunk,  which  opens  into  the  subclavian. 


Fig.  156. — Embryo  of  13  mm.  showing  the  Mode  of  Development  of  the  In- 
ternal Mammary  and  Deep  Epigastric  Arteries. — {Mall.) 

The  intercostals  and  their  abdominal  representatives,  the 
lumbars  and  ihacs,  also  give  rise  to  longitudinal  anastomosing 
twigs  near  their  ventral  ends  (Fig.  156),  and  these  increasing  in 
size  give  rise  to  the  internal  mammary  and  inferior  epigastric 
arteries,  which  together  form  continuous  stems  extending  from 
the  subclavians  to  the  external  iliacs  in  the  ventral  abdominal 
walls.  The  superficial  epigastrics  and  other  secondary  longitudi- 
nal vessels  are  formed  in  a  similar  manner. 


252  DEVELOPMENT    OF    ARTERIES    OF   LIMBS 

The  Development  of  the  Arteries  of  the  Limbs. — The  earliest 
stages  in  the  development  of  the  Umb  arteries  are  unknown  in  man, 
but  it  has  been  found  that  in  the  mouse  the  primary  supply  of  the 
anterior  limb  bud  is  from  five  branches  arising  from  the  sides  of 
the  aorta.  These  anastomose  to  form  a  plexus  from  which  later 
a  single  stem,  the  subclavian  artery,  is  elaborated,  occupying  the 
position  of  the  seventh  cervical  segmental  vessel,  the  remaining 
branches  of  the  plexus  having  disappeared.  The  common  iliac 
artery  similarly  represents  the  fifth  lumbar  segmental  artery, 
but  whether  or  not  it  also  is  elaborated  from  a  plexus  is  as  yet 
unknown. 

The  later  history  of  the  limb  arteries  is  also  but  imperfectly 
known  and  one  must  rely  largely  upon  the  facts  of  comparative 
anatomy  and  on  the  anomalies  that  occur  in  the  adult  for  indica- 
tions of  what  the  development  is  likely  to  be.  The  comparative 
evidence  indicates  the  existence  of  several  stages  in  the  develop- 
ment of  the  Hmb  vessels,  and  so  far  as  embryological  observations 
go  they  confirm  the  conclusions  drawn  from  this  source,  although 
the  various  stages  show  apparently  a  great  amount  of  overlapping 
owing  to  a  concentration  of  the  developmental  stages.  In  the 
simplest  arrangement  the  subclavian  is  continued  as  a  single  trunk 
along  the  axis  of  the  limb  as  far  as  the  carpus,  where  it  divides  into 
digital  branches  for  the  fingers.  In  its  course  through  the  fore- 
arm it  Hes  in  the  interval  between  the  radius  and  ulna,  resting  on 
the  interosseous  membrane,  and  in  this  part  of  its  course  it  may  be 
termed  the  arteria  interossea.  In  the  second  stage  a  new  artery 
accompanying  the  median  nerve  appears,  arising  from  the  main 
stem  or  brachial  artery  a  little  below  the  elbow-joint.  This  may 
be  termed  the  arteria  mediana,  and  as  it  develops  the  arteria  inter- 
ossea gradually  diminishes  in  size,  becoming  finally  the  small  volar 
interosseous  artery  of  the  adult  (Fig.  157),  and  the  median,  uniting 
with  its  lower  end,  takes  from  it  the  digital  branches  and  becomes 
the  principal  stem  of  the  forearm. 

A  third  stage  is  then  ushered  in  by  the  appearance  of  a  branch 
from  the  brachial  which  forms  the  arteria  ulnaris,  and  this,  passing 
down  the  ulnar  side  of  the  forearm,  unites  at  the  wrist  with  the 


DEVELOPMENT    OF    ARTERIES    OF   LIMBS 


253 


median  to  form  a  superficial  palmar  arch  from  which  the  digital 
branches  arise.     A  fourth  stage  is  marked  by  the  diminution  of  the- 
median  artery  until  it  finally  appears  to  be  a  small  branch  of  the 
interosseous,  and  at  the  same  time  there  develops  from  the  bra- 
chial, at  about  the  middle  of  the  upper  arm,  what  is  known  as  the 


A 


Fig.  157. — Diagrams  showing  an  Early  and  a  late  Stage  in  the  Development 
OF  THE  Arteries  of  the  Arm. 
h.  Brachial;  i,  interosseous;  rw,  median;  r,  radial;  rs,  superficial  radial;   u,  ulnar. 

arteria  radialis  super ficialis  (Fig.  157,  rs).  This  extends  down  the 
radial  side  of  the  forearm,  following  the  course  of  the  radial  nerve, 
and  at  the  wrist  passes  upon  the  dorsal  surface  of  the  hand  to  form 
the  dorsal  digital  arteries  of  the  thumb  and  index  finger.  At  first 
this  artery  takes  no  part  in  the  formation  of  the  palmar  arches, 
but  later  it  gives  rise  to  the  superficial  volar  branch,  which  usually 
unites  with  the  superficial  arch,  while  from  its  dorsal  portion  a 


254 


DEVELOPMENT    OF    ARTERIES    OF   LIMBS 


perforating  branch  develops  which  passes  between  the  first  and 
second  metacarpal  bones  and  unites  with  a  deep  branch  of  the 
ulnar  to  form  the  deep  arch.  The  fifth  or  adult  stage  is  reached 
by  the  development  from  the  brachial  below  the  elbow  of  branch 
(Fig.  157,  r)  which  passes  downward  and  outward  to  unite  with 
the  superficial  radial,  whereupon  the  upper  portion  of  that  artery 
degenerates  until  it  is  represented  only  by  a  branch  to  the  biceps 


•/ 


^gi 


e 


Fig.  158. — Diagram  illustrating  the  Development  of  the  Arteries  of  the 
Leg.     For  the  sake  of  Simplicity  the  Femoral  Rete  is  Omitted. 
(After  Senior) . 
ci.  Inferior;  cm,  middle;  cs,  superior  communicating  branch;  dp,  dorsal  plexus; 
/,  femoral;  gi,  inferior  gluteal;  I,  interosseous;  P.popliteus  muscle;  pc,  perforating 
erural;  pe,  peroneal;  pf,  profunda  femoris;  po,  popliteal;  pp,  plantar  plexus;  ps, 
superficial  peroneal;  pi,  peforating  tarsal;  s,  sciatic;  ta,  anterior  tibial;  tp,  posterior 
tibial. 

muscle  (Schwalbe),  while  the  lower  portion  persists  as  the  adult 

radial. 

The  various  anomalies  seen  in  the  arteries  of  the  forearm  are,  as  a 
rule,  due  to  the  more  or  less  complete  persistence  of  one  or  other  of  the 


DEVELOPMENT    OF   ARTERIES    OF    LIMBS  255 

stages  described  above,  what  is  described,  for  instance,  as  the  high 
branching  of  the  brachial  being  the  persistence  of  the  superficial  radial^ 

In  the  leg  there  is  a  noticeable  difference  in  the  arrangement 
of  the  arteries  from  what  occurs  in  the  arm,  in  that  the  principal 
artery  of  the  thigh,  the  femoral,  does  not  accompany  the  principal 
nerve,  the  sciatic.  This  condition  and  the  adult  arrangement  of 
the  crural  vessels  have  been  found  to  be  the  result  of  a  number  of 
somewhat  complicated  changes  (Senior),  the  more  important  of 
which  are  diagrammatically  represented  in  Fig.  1 58.  In  the  simplest 
stage,  which  is  to  be  seen  in  embryos  of  8.0-10.0  mm.,  a  single 
artery  extends  down  the  back  of  the  leg,  passing  anterior  to  the 
popliteus  muscle  and  thence  being  continued  down  the  crus  on  the 
interosseous  membrane,  to  terminate  in  a  plantar  network,  a 
branch  (Fig.  158  ^,  pt)  passing  through  the  tarsus  to  join  a  dorsal 
network.  The  upper  part  of  this  artery  may  be  termed  the  sciatic, 
while  its  crural  portion  may  be  spoken  of  as  the  interrosseous. 
The  femoral  at  this  stage  is  represented  by  a  network  of  vessels, 
the  femoral  reie,  which  extends  throughout  the  entire  length  of 
the  thigh  and  with  which  the  sciatic  communicates  by  a  branch 
passing  between  the  adductor  magnus  and  the  femur  (Fig. 
158,  B,  Cs), 

In  a  later  stage  the  superficial  femoral  and  the  profunda 
femoris  differentiate  from  the  femoral  rete,  the  former  being 
continuous  with  the  branch  of  the  sciatic  that  passes  through 
the  adductor  magnus.  From  the  sciatic  a  branch  (po),  which 
becomes  the  popliteal  artey  of  the  adult,  passes  down  over  the 
posterior  surface  of  the  popliteus,  immediately  below  that  muscle 
sending  a  branch  (cm)  to  communicate  with  the  interosseous, 
and  divides  a  little  lower  down  to  form  two  vessels,  one  of  which 
represents  the  posterior  tibial  (/^), while  the  other  may  be  termed 
the  superficial  peroneal  (ps).  From  this  last  a  communicating 
branch  (ci),  extends  downwards  to  join  the  lower  part  of  the 
interosseous,  from  whose  upper  part  a  branch  (pc)  passes  forward 
through  the  interosseous  membrane  and  thence  down  the  crus 
as  the  anterior  tibial  (ta)  to  join  the  dorsal  network.  The  plantar 
network  is  now  connected  with  the  interosseous,  the  superficial 


256  DEVELOPMENT   OF   THE  VENOUS    SYSTEM 

peroneal  and  the  posterior  tibial,  but  the  perforating  tarsal 
branch  which  united  it  with  the  dorsal  network  has  disappeared. 

This  represents  a  condition  from  which  the  adult  arrangement 
is  formed  by  the  disappearance  of  certain  vessels  (Fig.  158,  C). 
Almost  the  whole  of  the  sciatic  vanishes,  its  uppermost  portion 
persisting,  however,  to  form  the  inferior  gluteal  (gi)  and  the 
branch  of  that  vessel  which  accompanies  the  sciatic  nerve,  while 
a  small  portion  of  it,  just  where  it  joined  the  perforating  crural 
branch,  is  retained  to  form  the  medial  articular  artery  of  the  knee. 
The  upper  part  of  the  interosseous  also  disappears,  its  lower  part 
persisting  as  a  portion  of  the  adult  peroneal  (pe),  the  upper  part 
of  which  is  formed  by  what  was  the  communicating  branch  (ci) 
between  the  interosseous  and  the  superficial  peroneal,  this  latter 
artery  vanishing.  The  terminal  lateral  calcaneal  portion  of  the 
peroneal  is  a  new  formation  and  a  perforating  branch  from  the 
interosseous  passes  forward,  through  the  lower  part  of  the  inter- 
osseous membrane,  to  join  the  anterior  tibial.  The  dorsalis  pedis 
and  its  branches  are  differentiated  from  the  original  dorsal  network, 
while  the  plantar  arteries  are  similarly  derived  from  the  plantar 
network. 

The  Development  of  the  Venous  System. — The  earliest  veins 
to  develop  are  those  which  accompany  the  first-formed  arteries, 
the  umbilicals,  but  it  will  be  more  convenient  to  consider  first 
the  veins  which  carry  the  blood  from  the  body  of  the  embryo  back 
to  the  heart.  These  make  their  appearance,  while  the  heart  is 
still  in  the  pharyngeal  region,  as  two  pairs  of  longitudinal  trunks, 
the  anterior  and  posterior  cardinal  veins,  into  which  lateral  branches, 
arranged  more  or  less  segmentally,  open.  The  anterior  cardinals 
appear  somewhat  earlier  than  the  posterior  and  form  the  internal 
jugular  veins  of  adult  anatomy.  In  the  head  each  vein  passes 
along  the  side  of  the  brain  as  the  vena  capitis  prima,  passing  medial 
to  the  root  of  the  trigeminus  but  lateral  to  the  origins  of  the  more 
posterior  cranial  nerves  and  receiving  aff erents  from  three  plexuses 
which  extend  dorsally  over  the  walls  of  the  brain  iii  the  substance 
of  the  dura  mater  (Fig.  159  and  160,  A).  In  embryos  of  about 
20  mm.  the  anterior  and  middle  plexuses  have  united  (Fig.  160,  C) 


DEVELOPMENT    OF   THE   VENOUS    SYSTEM 


257 


and  have  developed  a  new  pathway  for  their  discharge,  which 
passes  backwards  dorsal  to  the!  ear  capsule  to  join  the  posterior 
plexus,  through  which  it  reaclfes  the  jugular  foramen.  At  the 
same  time  the  posterior  portion  of  the  vena  capitis  prima  dis- 
appears (Fig.  160,  B  and  C),  that  portion  of  it,  however,  which 
passes  medial  to  the  trigeminus  root  persisting,  since  it  receives 


an  fjcl  pcv 


Fig.  159. — Reconstruction  of  the  Head  of  a  Human  Fmbryo  of  9  mm.  showing 

THE  Cerebral  Veins. 
acv,  Anterior  cerebral  vein;  au,  auditory  vesicle;  cs,  cavernous  sinus;  fa,  facial 
nerve;  mcv,  middle  cerebral  vein;  pcv,  posterior  cerebral  vein;  tr,  trigeminal  nerve; 
vcv,  lateral  cerebral  vein. — (Mall.) 

from  in  front  the  opthalmic  vein  which  has  developed  from  the 
more  anterior  portions  of  the  anterior  plexus.  This  persisting 
portion  of  the  vena  capitis  prima  becomes  the  cavernous  sinus 
of  the  adult  and  now  drains  into  the  trunk  that  passes  dorsal  to 
the  ear  capsule,  by  a  vessel  which  represents  the  superior  petrosal 
sinus  of  the  adult.  Later  the  superior  sagittal  sinus  is  differ- 
entiated from  the  dorsal  portions  of  the  anterior  plexuses  (Fig. 

17 


258 


DEVELOPMENT    OF   THE  VENOUS    SYSTEM 


PLEXUS.  MECXALIS 


PLEXUS  MEStALIS 


B 


PLEXUS  ANT 


BIN.  RECTUS 
SIN.  SAGITTALIS  SUP 


PLEXUS   ANT 


SIN.  TRANSVERSUS 


SIN.  SAGnTALIS  SUP 


SIN.  SAGITTALIS 

SUP. 

^ 

"% 

SIN.  RECTUS 

f 

»^ 

CONFLUENS 

SINUUM 

/W__ 

W 

\  SIN.  TRANS 

^T^""^?^ 

"^f^ 

) 

.IN.  cavern;^     /^ 

> 

(pars  SIGMOID, 

S   PETROSINF.         / 

\W\ 

7 

Ts.  PETROS  SUP 

ji  / 

/ 

y  i 

ri  1 

INT 

SIN.  SAGITTALIS 


E  F 

Fig.    i6o.— Six  Stages  in  the  Development  of  the  Sinuses  of  the  Dura 

Mater.     {SireeUr). 


DEVELOPMENT    OF   THE  VENOUS    SYSTEM  250 

1 60  D),  while  the  straight  and  inferior  sagittal  sinuses  are  elabor- 
ated from  those  portions  of  the  plexuses  which  extend  down  be- 
tween the  two  cerebral  hemispheres  in  the  falx  cerebri;  and  since" 
the  superior  sagittal  and  straight  sinuses  open  into  the  trunk 
which  passes  dorsal  to  the  ear  capsule,  it  is  now  clear  that  this 
trunk  represents  the  transverse  sinus  of  adult  anatomy.  The 
essential  features  of  the  adult  arrangement  are  now  completed 
by  the  formation  of  the  inferior  petrosal  sinus  (Fig.  160  D),  this 
being  practically  a  reconstitution  of  the  posterior  portion  of  the 
original  vena  capitis  prima,  and  the  various  parts  of  the  cerebral 
system  of  veins  are  brought  into  their  adult  relations  by  the 
straightening  out  of  the  nape  bend  and  by  the  continued  growth  of 
the  cerebral  hemispheres.     (Fig.  160,  E  and  F). 

Passing  backward  from  the  jugular  foramen  the  internal  jugu- 
lar veins  unite  with  the  posterior  cardinals  to  form  on  each  side  a 
common  trunk,  the  ductus  Cuvieri,  which  passing  transversely  to- 
ward the  median  Hne,  opens  into  the  side  of  the  sinus  venosus. 
So  long  as  the  heart  retains  its  original  position  in  the  pharyngeal 
region  the  jugular  is  a  short  trunk  receiving  lateral  veins  only  from 
the  uppermost  segments  of  the  neck  and  from  the  occipital  seg- 
ments, the  remaining  segmental  veins  opening  into  the  inferior 
cardinals.  As  the  heart  recedes,  however,  the  jugulars  become 
more  and  more  elongated  and  the  cervical  lateral  veins  shift  their 
communication  from  the  cardinals  to  the  jugulars,  until,  when  the 
subclavians  have  thus  shifted,  the  jugulars  become  much  larger 
than  the  cardinals.  When  the  sinus  venosus  is  absorbed  into  the 
wall  of  the  right  auricle,  the  course  of  the  left  Cuvierian  duct  be- 
comes a  little  longer  than  that  of  the  right,  and  from  the  left  jugu- 
lar, at  the  point  where  it  is  joined  by  the  left  subclavian,  a  branch 
arises  which  extends  obliquely  across  to  join  the  right  jugular, 
forming  the  left  innominate  vein.  When  this  is  established,  the 
connection  between  the  left  jugular  and  Cuvierian  duct  is  dis- 
solved, the  blood  from  the  left  side  of  the  head  and  neck  and  from 
the  left  subclavian  vein  passing  over  to  empty  into  the  right  jugular 
whose  lower  end,  together  with  the  right  Cuvierian  duct,  thus  be- 
comes the  superior  vena  cava.     The  left  Cuvierian  duct  persists 


26o 


DEVELOPMENT    OF   THE  VENOUS    SYSTEM 


forming  with  the  left  horn  of  the  sinus  venosus  the  coronary  sinus 
(Fig.  i6i). 

The  external  jugular  vein  develops  somewhat  later  than  the 
internal.  The  facial  vein,  which  primarily  forms  the  principal 
affluent  of  this  stem,  passes  at  first  into  the  skull  along  with  the 
fifth  nerve  and  communicates  with  the  internal  jugular  system,  but 
later  this  original  communication  is  broken  and  the  facial  vein, 
uniting  with  other  superficial  veins,  passes  over  the  jaw  and  ex- 
tends down  the  neck  as  the  external  jugular.    Later  still  the  facial 


Fig.   i6i, — Diagrams  showing  the  Development  of  the  Superior  Vena  Cava. 
a,  Azygos  vein;  cs,  coronary  sinus;  ej,  external  jugular;  h,  hepatic  vein,  ij,  internal 
jugular;  inr  and  inl,  right  and  left  innominate  veins;  s,  subclavian;  vci  and  vcs,  in- 
ferior and  superior  venae  cavae. 

anastomoses  with  the  ophthalmic  at  the  inner  angle  of  the  eye  and 
also  makes  connections  with  the  internal  jugular  just  after  it  has 
crossed  the  jaw,  and  so  the  adult  condition  is  acquired. 

It  is  interesting  to  note  that  in  many  of  the  lower  mammals  the 
external  jugular  becomes  of  much  greater  importance  than  the  internal, 
the  latter  in  some  forms,  indeed,  eventually  disappearing  and  the  blood 
from  the  interior  of  the  skull  emptying  by  means  of  anastomoses  which 
have  developed  into  the  external  jugular  system.  In  man  the  primi- 
tive condition  is  retained,  but  indications  of  a  transference  of  the 
intracranial  blood  to  the  external  jugular  are  seen  in  the  emissary  veins. 

The  posterior  cardinal  veins,  or,  as  they  may  more  simply  be 
termed,  the  cardinals,  extend  backward  from  their  union  with  the 


DEVELOPMENT   OF   THE   VENOUS    SYSTEM  261 

jugulars  along  the  sides  of  the  vertebral  column,  receiving  veins 
from  the  mesentery  and  also  from  the  various  lateral  segmental 
veins  of  the  neck  and  trunk  regions,  with  the  exception  of  that  or 
the  first  cervical  segment  which  opens  into  the  jugular.  Later, 
however,  as  already  described  (p.  259),  the  cervical  veins  shift  to 
the  jugulars,  as  do  also  the  first  and  second  thoracic  (intercostal) 
veins,  but  the  remaining  intercostals,  together  with  the  lumbars 
and  sacrals,  continue  to  open  into  the  cardinals.  In  addition,  the 
cardinals  receive  in  early  stages  the  veins  from  the  primitive  kid- 
neys (mesonephros),  which  are  exceptionally  large  in  the  human 
embryo,  but  as  they  become  replaced  later  on  by  the  permanent 
kidneys  (metanephros)  their  afferent  veins  undergo  a  reduction  in 
number  and  size,  and  this,  together  with  the  shifting  of  the  upper 
lateral  veins,  produces  a  marked  diminution  in  the  size  of  the  car- 
dinals. The  changes  by  which  they  acquire  their  final  arrange- 
ment are,  however,  so  intimately  associated  with  the  development 
of  the  inferior  vena  cava  that  their  description  may  be  conven- 
iently postponed  until  the  history  of  the  vitelline  and  umbilical 
veins  has  been  presented. 

The  vitelline  veins  are  two  in  number,  a  right  and  a  left,  and  pass 
in  along  the  yolk-stalk  until  they  reach  the  embryonic  intestine, 
along  the  sides  of  which  they  pass  forward  to  unite  with  the  corre- 
sponding umbilical  veins.  These  are  represented  in  the  belly- 
stalk  by  a  single  venous  trunk  which,  when  it  reaches  the  body  of 
the  embryo,  divides  into  two  stems  which  pass  forward,  one  on 
each  side  of  the  umbilicus,  and  thence  on  each  side  of  the  median 
line  of  the  ventral  abdominal  wall,  to  form  with  the  corresponding 
vitelline  veins  common  trunks  which  open  into  the  ductus  Cuvieri. 
As  the  liver  develops  it  comes  into  intimate  relation  with  the  vitel- 
line veins,  which  receive  numerous  branches  from  its  substance 
and,  indeed,  seem  to  break  up  into  a  network  (Fig.  162,  A)  tra- 
versing the  liver  substance  and  uniting  again  to  form  two  stems 
which  represent  the  original  continuations  of  the  vitellines. 
From  the  point  where  the  common  trunk  formed  by  the  right  vitel- 
line and  umbilical  veins  opens  into  the  Cuverian  duct  a  new  vein 
develops,  passing  downward  and  to  the  left  to  unite  with  the  left 


262 


DEVELOPMENT    OF    THE   VENOUS    SYSTEM 


vitelline;  this  s  the  ductus  venosus  (Fig.  162,  B,  D.V.A.).  In  the 
meantime  three  cross-connections  have  developed  between  the  two 
vitelline  veins,  two  of  which  pass  ventral  and  the  other  dorsal  to 
the  intestine,  so  that  the  latter  is  surrounded  by  two  venous  loops 
(Fig.  163,  ^),  and  a  connection  is  developed  between  each  umbilical 
vein  and  the  corresponding  vitelline  (Fig.  162,  B),  that  of  the  left 
side  being  the  larger  and  uniting  with  the  vitelHne  just  where 
it  is  joined  by  the  ductus  venosus,  so  as  to  seem  to  be  the  continua- 
tion of  this  vessel  (Fig.   162,  C)./^When  these  connections  are 


jDC, 


DC 


Vus. 


Vo.m.s 


DC 


~2?.KA 


PTus 


U^v^ 


Vud 


VoTTl^d.  VOTJVS. 


Fig.   162. — Diagrams  Illustrating  the  Transformations  of  the  Vitelline 

AND  Umbilical  Veins. 
D.5,  Ductus  Cuvieri;  D.V.A,  ductus  venosus;  V .o.m.d  and  V .o.m.s,  right  and  left 
vitelline  veins;  V .u.d  and  V .u.s,  right  and  left  umbilical  veins. — (Hochstetter.) 

complete,  the  upper  portions  of  the  umbilical  veins  degenerate  (Fig. 
163),  and  now  the  right  side  of  the  lower  of  the  two  vitelline  loops 
which  surround  the  intestine  disappears,  as  does  also  that  portion 
of  the  left  side  of  the  upper  loop  which  intervenes  between  the 
middle  cross-connection  and  the  ductus  venosus,  and  so  there  is 
formed  from  the  vitelline  veins  the  vena  portce. 

While  these  changes  have  been  progressing  the  right  umbilical 
vein,  originally  the  larger  of  the  two  (Fig.  162,  A  and  B,  V.u.d.), 
has  become  very  much  reduced  in  size  and,  losing  its  connection 
with  the  left  vein  at  the  umbilicus,  forms  a  vein  of  the  ventral  ab- 


DEVELOPMENT    OF    THE   VENOUS    SYSTEM 


263 


dominal  wall  in  which  the  blood  now  flows  from  above  downward. 
The  left  umbilical  now  forms  the  only  route  for  the  return  of  blood 
from  the  placenta,  and  appears  to  be  the  direct  continuation  of  the 
ductus  venosus  (Fig.  163,  C),  into  which  open  the  hepatic  veins ^  re- 
turning the  blood  distributed  by  the  portal  vein  to  the  substance  of 
the  liver. 

Returning  now  to  the  posterior  cardinal  veins,  it  has  been  found 
that  in  the  rabbit  the  branches  which  come  to  them  from  the 
mesentery  anastomose  longitudinally  to  form  a  vessel  lying  parallel 


Fig.  163. — A,  the  Venous  Trunks  of  an  Embryo  of  5  mm.  seen  from  the 
Ventral  Surface;  B,  Diagram  Illustrating  the  Transformation  to  the 
Adult  Cono  tion. 

Vcd  and  Vcs,  Right  and  left  superior  venae  cavas;  Vj,  jugular  vein;  V.om,  vitelline 
vein;  Vp,  vena  portae;  Vu,  umbilical  vein  (lower  part);  Vu',  umbilical  vein  (upper 
part);  Vud  and  Vus,  right  and  left  umbilical  veins  (lower  parts). — (His.) 


and  slightly  ventral  to  each  cardinal.  These  may  be  termed  the 
subcardinal  veins  (Lewis),  and  in  their  earliest  condition  they  open 
at  either  end-4nto  the  corresponding  cardinal,  with  which  they  are 
also  united  by  numerous  cross-branches.  Later,  in  rabbits  of  8.8 
mm.,  these  cross-branches  begin  to  disappear  and  give  place  to  a 
large  cross-branch  situated  immediately  below  the  origin  of  the 
superior  mesenteric  artery,  and  at  the  same  point  a  cross  branch 
between  the  two  subcardinals  also  develops.     The  portion  of  the 


264 


DEVELOPMENT    OF   THE   VENOUS    SYSTEM 


right  subcardinal  which  is  anterior  to  the  cross-connection  now 
rapidly  enlarges  and  unites  with  the  ductus  venosus  about  where 
the  hepatic  veins  open  into  that  vessel  (Fig.  164  A),  and  the  por- 
tion of  each  posterior  cardinal  immediately  above  the  entrance  of 
the  renal  veins  degenerates,  so  that  all  the  blood  received  by  the 
posterior  portions  of  the  cardinals  is  returned  to  the  heart  by  way 
of  the  right  subcardinal,  its  cross-connections,  and  the  upper  part 
of  the  ductus  venosus. 


Fig.  164. — Diagrams  Illustrating  the  Development  of  the  Inferior  Vena 

Cava. 
The  cardinal  veins  and  ductus  venosus  are  black,  the  subcardinal  system  blue, 
and  the  supracardinal  yellow,     cs,  coronary  sinus;  dv,  ductus  venosus;  il,  iliac  vein; 
r,  renal;  s,  internal  spermatic;  scl,  subclavian;  sr,  suprarenal;  va,  azygos;vha,  hemi- 
azygos; vi,  innominate;  vj,  internal  jugular. 

When  this  is  accomplished  the  lower  portions  of  the  subcardi- 
nals  disappear,  while  the  portions  above  the  large  cross-connec- 
tion persist,  greatly  diminished  in  size,  as  the  suprarenal  veins 
(Fig.  164,  B). 

In  the  early  stages  the  veins  which  drain  the  posterior  abdomi- 
nal walls  empty  into  the  posterior  cardinals,  and  later  they  form,  in 


DEVFXOPMENT    OF   THE  VENOUS    SYSTEM  265 

the  region  of  the  kidney  on  each  side,  a  longitudinal  anastomosis 
which  opens  at  either  extremity  into  the  posterior  cardinal.  The_ 
ureter  thus  becomes  surrounded  by  a  venous  ring,  the  dorsal  limb 
of  which  is  formed  by  the  new  longitudinal  anastomosis,  which  has 
been  termed  the  supracardinal  vein  (McClure  and  Huntington) 
while  the  ventral  Kmb  is  formed  by  a  portion  of  the  posterior 
cardinal  (Fig.  164,  B).  Still  later  the  ventral  limb  of  the  loop 
disappears  and  the  dorsal  supracardinal  Hmb  replaces  a  portion 
of  the  more  primitive  posterior  cardinal.  An  anastomosis  now 
develops  between  the  right  and  left  cardinals  at  the  point  where 
the  iliac  veins  open  into  them  (Fig.  163,  -B),  and  the  portion  of  the 
left  cardinal  which  intervenes  between  this  anastomosis  and  the 
entrance  of  the  internal  spermatic  vein  disappears,  the  remainder 
of  it,  as  far  forward  as  the  renal  vein,  persisting  as  the  upper  part 
of  the  left  internal  spermatic  vein,  which  thus  comes  to  open  into 
the  renal  vein  instead  of  into  the  vena  cava  as  does  the  corre- 
sponding vein  of  the  right  side  of  the  body  (Fig.  164,  C,j).  The 
renal  veins  originally  open  into  the  cardinals  at  the  point  where 
these  are  joined  by  the  large  cross-connection,  and  when  the  lower 
part  of  the  left  cardinal  disappears,  this  cross-connection  forms 
the  proximal  part  of  the  left  renal  vein,  which  consequently 
receives  the  left  suprarenal  (Fig.  164,  C). 

The  observations  upon  which  the  above  description  is  based 
have  been  made  chiefly  upon  the  rabbit  and  pig,  but  it  seems  pro- 
bable from  the  partial  observations  that  have  been  made  that  sim- 
lar  changes  occur  also  in  the  human  embryo.  It  will  be  noted 
from  what  has  been  said  that  the  inferior  vena  cava  is  a  composite 
vessel,  consisting  of  at  least  four  elements:  (i)  the  proximal  part 
of  the  ductus  venosus;  (2)  the  anterior  part  of  the  right  sub- 
cardinal;  (3)  the  right  supracardinal;  and  (4)  the  posterior  part 
of  the  right  cardinal. 

The  complicated  development  of  the  inferior  vena  cava  naturally 
gives  rise  to  numerous  anomalies  of  the  vein  due  to  inhibitions  of  its 
development.  These  anomalies  affect  especially  the  post-renal  portion 
a  persistence  of  both  cardinals  (interpreting  the  conditions  in  the 
terms  of  what  occurs  in  the  rabbit)  giving  rise  to  a  double  post-renal 


266  DEVELOPMENT    OF    THE   VENOUS    SYSTEM 

cava,  or  a  persistence  of  the  left  cardinal  and  the  disappearance  of  the 
right  to  a  vena  cava  situated  on  the  left  side  of  the  vertebral  column 
and  crossing  to  the  right  by  way  of  the  left  renal  vein.  So,  too  the 
occurrence  of  accessory  renal  veins  passing  dorsal  to  the  ureter  is  ex- 
plicable on  the  supposition  that  they  represent  portions  of  the  supra- 
cardinal  system  of  veins. 

It  has  already  been  noted  that  the  portions  of  the  posterior 
cardinals  immediately  anterior  to  the  entrance  of  the  renal  veins 
disappear.  The  thoracic  portion  of  the  right  vein  persists,  how- 
ever, and  becomes  the  vena  azygos  of  the  adult,  while  the  upper 
portion  of  the  left  vein  sends  a  cross-branch  over  to  unite  with 
the  azygos  and  then  separates  from  the  coronary  sinus  to  form 
the  vena  hemiazygos.  At  least  this  is  what  is  described  as  occur- 
ring in  the  rabbit.  In  the  cat,  however,  only  the  very  uppermost 
portion  of  the  right  posterior  cardinal  persists  and  the  greater 
portion  of  the  azygos  and  perhaps  the  entire  hemizaygos  vein  is 
formed  from  the  prerenal  portions  of  the  supracardinal  veins,  the 
right  one  joining  on  to  the  small  persisting  upper  portion  of  the 
right  posterior  cardinal,  while  the  cross-connection  between  the 
hemiazygos  and  azygos  represents  one  of  the  originally  numerous 
cross-connections  between  the  supracardinals. 

The  ascending  lumbar  veins,  frequently  described  as  the  commence- 
ments of  the  azygos  veins,  are  in  reality  secondary  formations  de- 
veloped by  the  anastomoses  of  anteriorly  and  posteriorly  directed 
branches  of  the  lumbar  veins. 

The  Development  of  the  Veins  of  the  Limbs. — The  development 
of  the  limb  veins  of  the  human  embryos  requires  further  investiga- 
tion, but  from  a  comparison  of  what  is  known  with  what  has  been 
observed  in  rabbit  embryos  it  may  be  presumed  that  the  changes 
which  take  place  are  somewhat  as  follows:  In  the  anterior  ex- 
tremity the  blood  brought  to  the  limb  is  collected  by  a  vein  which 
passes  distally  along  the  radial  border  of  the  limb  bud,  around  its 
distal  border,  and  proximally  along  its  ulnar  border  to  open  into 
the  anterior  cardinal  vein;  this  is  the  primary  ulnar  vein.  Later  a 
second  vein  grows  out  from  the  external  jugular  along  the  radial 
border  of  the  limb,  representing  the  cephalic  vein  of  the  adult,  and 
on  its  appearance  the  digital  veins,  which  were  formed  from  the 


THE    FETAL    CIRCULATION  267 

primary  ulnar  vein,  becomes  connected  with  it,  and  the  distal 
portion  of  the  primary  ulnar  vein  disappears.  Its  proximal  por- 
tion persists,  however,  to  form  the  basilic  vein,  from  which  the 
brachial  vein  and  its  continuation,  the  ulnar  vein,  are  developed, 
while  the  radial  vein  develops  as  an  outgrowth  from  the  cephalic, 
which  at  an  early  stage  secures  an  opening  into  the  axillary  vein, 
its  original  communication  with  the  external  jugular  forming  the 
jugulo-cephalic  vein. 

In  the  lower  limb  a  primary  fibular  vein,  exactly  comparable  to 
the  primary  ulnar  of  the  arm,  surrounds  the  distal  border  of  the 
limb-bud  and  passes  up  its  fibular  border  to  open  with  the  poste- 
rior cardinal  vein.  The  further  development  in  the  lower  limb  dif- 
fers considerably,  however,  from  that  of  the  upper  limb.  From 
the  primary  fibular  vein  an  anterior  tibial  vein  grows  out,  which  re- 
ceives the  digital  branches  from  the  toes,  and  from  the  posterior 
cardinal,  anterior  to  the  point  where  the  primary  fibular  opens  into 
it,  a  vein  grows  down  the  tibial  side  of  the  leg,  forming  the  long 
saphenous  vein.  From  this  the  femoral  vein  is  formed  and  from  it 
the  posterior  tibial  vein  is  continued  down  the  leg.  An  anastomo- 
sis is  formed  between  the  femoral  and  the  primary  fibular  veins  at 
the  level  of  the  knee  and  the  proximal  portion  of  the  latter  vein 
then  becomes  greatly  reduced,  while  its  distal  portion  possibly 
persists  as  the  small  saphenous  vein  (Hochstetter). 

The  Pulmonary  Veins. — The  development  of  the  pulmonary 
veins  has  already  been  described  in  connection  with  the  develop- 
ment of  the  heart  (see  p.  235). 

The  Fetal  Circulation. — During  fetal  life  while  the  placenta  is 
the  sole  organ  in  which  occur  the  changes  in  the  blood  on  which  the 
nutrition  of  the  embryo  depends,  the  course  of  the  blood  is  neces- 
sarily somewhat  different  from  what  obtains  in  the  child  after 
birth.  Taking  the  placenta  as  the  starting-point,  the  blood  passes 
along  the  umbiHcal  vein  to  enter  the  body  of  the  fetus  at  the  umbili- 
cus, whence  it  passes  forward  in  the  free  edge  of  the  ventral  mesen- 
tery (see  p.  324)  until  it  reaches  the  liver.  Here,  owing  to  the 
anastomoses  between  the  umbiHcal  and  vitelHne  veins,  a  portion  of 
the  blood  traverses  the  substance  of  the  liver  to  open  by  the  hepat- 


268 


THE   FETAL    CIRCULATION 


ic  veins  into  the  inferior  vena  cava,  while  the  remainder  passes  on 
through  the  ductus  venosus  to  the  cava,  the  united  streams  open- 
ing into  the  right  atrium.  This  blood,  whose  purity  is  only 
slightly  reduced  by  mixture  with  the  blood  returning  from  the  in- 


-vJ; 


Fig.   165. — The  Fetal  Circulation. 
ao,  Aorta;  a.pu.,  pulmonary  artery;  au,  umbilical  artery;  da,  ductus  arteriosus; 
dv,  ductus  venosus;  int,  intestine;  vci  and  vsc,  inferior  and  superior  vena  cava;  vh, 
hepatic    vein;  vp,  vena  portae;  v.pu,  pulmonary  vein;  vu,  umbilical   vein. — (From 
Kollmann.) 

f erior  vena  cava,  is  prevented  from  passing  into  the  right  ventricle 
by  the  Eustachian  valve,  which  directs  it  to  the  foramen  ovale,  and 
through  this  it  passes  into  the  left  atrium,  thence  to  the  left 
ventricle,  and  so  out  by  the  systemic  aorta. 


THE    FETAL   CIRCULATION  269 

The  blood  which  has  been  sent  to  the  head,  neck,  and  upper 
extremities  is  returned  by  the  superior  vena  cava  also  into  the 
right  atrium,  but  this  descending  stream  opens  into  the  atrium  to 
right  of  the  annulus  of  Vieussens  (see  Fig.  143)  and  passes  directly 
to  the  right  ventricle  without  mingling  to  any  great  extent  with  the 
blood  returning  by  way  of  the  inferior  cava.  From  the  right  ven- 
tricle this  blood  passes  out  by  the  pulmonary  artery;  but  the  lungs 
at  this  period  are  collapsed  and  in  no  condition  to  receive  any  great 
amount  of  blood,  and  so  the  stream  passes  by  way  of  the  ductus 
arteriosus  into  the  systemic  aorta,  meeting  there  the  placental 
blood  just  below  the  point  where  the  left  subclavian  artery  is 
given  off.  From  this  point  onward  the  aorta  contains  only  mixed 
blood,  and  this  is  distributed  to  the  walls  of  the  thorax  and  ab- 
domen and  to  the  lungs  and  abdominal  viscera,  the  greater  part  of 
it,  however,  passing  off  in  the  hypogastric  arteries  and  so  out  again 
to  the  placenta. 


This  is  the  generally  accepted  account  of  the  fetal  circulation  and  it 
is  based  upon  the  idea  that  the  foramen  ovale  is  practically  a  con- 
nection between  the  inferior  vena  cava  and  the  left  atrium.  If  it  be 
correct  the  right  ventricle  receives  only  the  blood  returning  to  the 
heart  by  the  vena  cava  superior,  while  the  left  receives  all  that  returns 
by  the  inferior  vena  cava  together  with  what  returns  by  the  pulmonary 
veins.  One  would,  therefore,  expect  that  the  capacity  and  pressure 
of  the  right  ventricle  would  in  the  fetus  be  less  than  those  of  the  left. 
Pohlman,  who  has  recently  investigated  the  question  in  embryo  pigs, 
finds,  on  the  contrary,  that  the  capacities  and  pressures  of  the  two 
ventricles  are  equal  and  maintains  that  the  foramen  ovale  is  actually  a 
connection  between  the  two  atria.  That  is  to  say,  he  holds  that  there 
is  an  actual  mingling  of  the  blood  from  the  two  venae  cavae  in  the 
right  atrium,  whence  the  mixed  blood  passes  to  the  right  ventricle,  a 
certain  amount  of  it,  however,  passing  through  the  foramen  ovale  and 
so  to  the  left  ventricle  to  equalize  the  deficiency  that  would  otherwise 
exist  in  that  chamber  owing  to  the  small  amount  of  blood  returning  by 
the  pulmonary  veins.  According  to  this  view  there  would  be  no 
difference  in  the  quality  of  the  blood  distributed  to  different  portions 
of  the  body,  such  as  is  provided  for  by  the  current  theory;  all  the 
blood  leaving  the  heart  would  be  mixed  blood  and  in  favor  of  this 
view  is  the  fact  that  starch  granules  injected  into  either  the  superior 
or  the  inferior  vena  cava  in  living  pig  embryos  were  in  all  cases  re- 
covered from  both  sides  of  the  heart.  . 


270  DEVELOPMENT    OF    THE    LYMPHATIC    SYSTEM 

At  birth  the  lungs  at  once  assume  their  functions,  and  on  the 
cutting  of  the  umbilical  cord  all  communication  with  the  placenta 
ceases.  Shortly  after  birth  the  foramen  ovale  closes  more  or  less 
perfectly,  and  the  ductus  arteriosus  diminishes  in  size  as  the  pul- 
monary arteries  increase  and  becomes  eventually  converted  into  a 
fibrous  cord.  The  hypogastric  arteries  diminish  greatly,  and  after 
they  have  passed  the  bladder  are  also  reduced  to  fibrous  cords,  a 
fate  likewise  shared  by  the  umbilical  vein,  which  becomes  con- 
verted into  the  round  ligament  of  the  liver. 

The  Development  of  the  Lymphatic  System. — The  lymphatic 
system  is  associated  with  the  blood-vascular  system  both  in  its 
adult  condition  and  in  its  development.  Indeed,  at  one  stage  it 
is  virtually  a  part  of  the  blood-vascular  system,  being  represented 
by  capillary  networks  hardly  distinguishable  from  adjacent  blood 
capillaries,  containing  blood  like  these  and  being  connected  with 
neighboring  venous  trunks.  These  networks  are  developed  in 
definite  regions  of  the  body,  one  being  formed  in  relation  with  the 
proximal  portion  of  each  anterior  cardinal  (internal  jugular)  vein, 
another  pair  appearing  along  the  lines  of  the  iliac  veins,  while 
another,  unpaired,  develops  in  the  root  of  the  mesentery  along 
the  line  of  the  median  vein  draining  the  mesonephros  (see  p.  342). 

In  later  stages  the  vessels  forming  these  networks  dilate  and 
unite  together  to  form  sac-like  structures,  termed  lymph  sacs, 
which  are  accordingly  five  in  number,  i.e.,  two  jugular  (Fig.  166, 
ALH),  two  iliac  (Fig.  166,  PLH)  and  one  retroperitoneal  (Fig.  167 
Isr).  At  first  these  lymph  sacs  still  contain  blood  and  are  con- 
nected with  the  neighboring  venous  trunks,  but  later  they  evacuate 
their  blood  contents  and  separate  from  the  veins,  forming  inde- 
pendent sacs  lined  by  endothelium.  In  relation  with  these  as 
centers  the  remaining  portions  of  the  lymphatic  system,  the  tho- 
racic duct  and  the  peripheral  vessels,  develop,  the  sacs  themselves 
eventually  becoming  transformed  into  groups  of  lymphatic  nodes, 
the  jugular  ones,  however,  re-establishing  connections  between  the 
lymphatic  and  venous  systems  by  uniting  with  the  junctions  of  the 
jugular  and  subclavian  veins. 

With  regard  to  the  development  of  the  thoracic  duct  and  per- 


DEVELOPMENT   OF   THE    LYMPHATIC    SYSTEM 


271 


ipheral  vessels,  as  well  as  with  regard  to  the  first  formation  of  the 
primary  networks  from  which  the  lymph  sacs  develop,  two  dis-_ 
cordant  views  exist.  According  to  one  (Sabin,  Lewis)  the  net- 
works are  formed  by  the  union  of  a  number  of  outgrowths  from 


Fig.  166. — Diagrams  showing  the  Arrangement  of  the  Lymphatic  Vessels  in 
Pig  Embryos  of  (A)  20  mm.  and  (B)  40  mm. 
ACV,  Jugular  vein;  ADR,  suprarenal  body;  ALH,  jugular  lymph  sac;  Ao,  aorta? 
Arm  D,  deep  lymphatics  to  the  arm;  D,  diaphragm;  Du,  branches  to  duodenum; 
FV,  femoral  vein;  H,  branches  to  heart;  K,  kidney;  Leg  D,  deep  lymphatics  to  leg; 
Lu,  branches  to  lung;  MP,  branches  to  mesenteric  plexus;  CE,  branch  to  oesophagus; 
PCV,  cardinal  vein;  PLH,  posterior  lymph  sac;  RC,  cisterna  chyli;  RLD,  right 
lymphatic  duct;  ScV,  subclavian  vein;  SV,  sciatic  vein;  St,  branches  to  stomach;  TD, 
thoracic  duct;  WB,  Wolffian  body. —  (Sabin.) 


the  veins  and  the  peripheral  vessels  are  formed  by  a  process  of 
budding  from  the  lymph  sacs,  outgrowths  of  the  endothelium  of 


272 


DEVELOPMENT    OF   THE    LYMPHATIC    SYSTEM 


these  radiating  into  the  surrounding  mesenchyme.  From  the 
jugular  sacs  are  formed  the  vessels  which  drain  the  upper  half  of 
each  side  of  the  body  and  the  arms,  from  the  iliac  sacs  those  drain- 
ing the  walls  of  the  lower  half  of  each  side  of  the  body,  the  perma- 
nent kidneys  and  the  legs,  and  from  the  retroperitoneal  sac  the 
vessels  draining  the  remaining  abdominal  and  pelvic  viscera.     The 


Fig.  167. — Diagram  of  the  Posterior  Portion  of  the  Body  of  a  Human 
Embryo  of  23  mm.,  showing  the  Relations  of  the  Retroperitoneal  Lymph 
Sac  and  the  Cisterna  Chyli  to  the  Veins. 

Am,  Superior  mesenteric  artery;  Ao,  aorta;  Cc,  cisterna  chyli;  /5^,  retroperitoneal 
lymph  sac;  S,  suprarenal  body;  Va,  vena  azygos;  Vci,  vena  cava  inferior;  vh,  first 
lumbar  vertebra;  vsu  first  sacral  vertebra. — (After  Sabin.) 

thoracic  duct  is  formed  by  the  union  of  two  originally  distinct 
portions,  one,  a  downward  growth  from  the  left  jugular  sac  and 
the  other  a  network  formed  from  outgrowths  from  the  retroperi- 
toneal sac.  This  network  lies  behind  the  aorta  and  gives  rise  to 
the  cisterna  chyli  and  the  greater  portion  of  the  thoracic  duct,  the 
frequent  duplication  of  this  structure,  especially  in  its|  lower  por- 
tion, being  thus  readily  understood  from  its  mode  of  development. 


DEVELOPMENT   OF   THE    LYMPHATIC    SYSTEM 


273 


According  to  this  view  the  endothelium  lining  the  lymphatic 
vessels  is  derived  directly  from  that  lining  the  blood-vessels  and- 
the  development  of  the  peripheral  lymphatics  is  by  a  centrifugal 
growth  from  the  lymph  sacs.  According  to  the  opposing  view 
(Huntington,  McClure)  the  lymphatics  in  their  initial  stage  are 
independent  of  the  blood-vessels,  appearing  as  a  number  of  inter- 
cellular clefts  in  the  mesenchyme  along  the  line  of  venous  trunks. 
These  clefts  become  lined  by  an  endo- 
thelium, blood  corpuscles  from  adjacent 
blood-islands  make  their  way  into  them 
and  gradually  the  clefts  unite  together 
to  form  a  capillary  network  which 
makes  connections  with  the  neighboring 
vein.  In  this  way  are  formed  the  pri- 
mary networks  from  which  the  lymph 
sacs  develop,  and  the  same  process  leads 
to  the  formation  of  the  thoracic  duct 
and  the  peripheral  lymphatics,  the  duct, 
for  example,  arising  by  the  union  of  a 
series  of  clefts  in  the  mesenchyme  along 
the  line  of  the  left  posterior  cardinal 
vein,  the  canal  so  formed  eventually 
uniting  with  the  left  jugular  lymph  sac. 
On  this  view  the  primary  lymphatic  net- 
works serve  to  convey  to  the  main 
venous  trunks  the  blood  which  is  being 
formed  in  isolated  blood-islands  through- 
out the  mesenchyme,  and  it  is  only  secondarily,  on  the  cessation 
of  the  haematopoietic  function  of  the  mesenchyme,  that  they  take 
on  the  lymphatic  function.  Their  endothelium  arises  quite  inde- 
pendently of  that  of  the  blood-vascular  system  and  the  mode  of 
growth  of  the  vessels  is,  in  a  sense,  centripetal  toward  the  lymph 
sacs. 

Lymph  nodes  have  not  been  observed  in  human  embryos  until 
toward  the  end  of  the  third  month  of  development,  but  they 
appear  in  pig  embryos  of  3  cm.     Their  unit  of  structure   is  a 


Fig.  168. — Diagram  of  a 
Primary  Lymph  Node  of  an 
Embryo  Pig  of  8  cm. 

a.  Artery;  aid,  afferent 
lymph  duct;  eld,  efferent 
lymph  duct;  /,  follicle. — 
iSahin.) 


18 


274 


DEVELOPMENT    OF    THE    LYMPHATIC    SYSTEM 


blood-vessel,  breaking  up  at  its  termination  into  a  leash  of  capil- 
laries, around  which  a  condensation  of  lymphocytes  occurs  in  the 
mesenchyme.  A  structure  of  this  kind  forms  what  is  termed  a 
lymphoid  follicle  and  may  exist,  even  in  this  simple  condition,  in 
the  adult.  More  frequently,  however,  there  are  associated  with 
the  follicle  lymphatic  vessels,  or  rather  the  follicle  develops  in  a 
network  of  lymphatic  vessels,  which  become  an  investment  of  the 


Fig.  169. — Developing  H^molymph  Node. 
be,  central  blood-vessel;  hh,  blood-vessel  at  hilus;  ps,  peripheral  blood  sinus. 
from  Morris'  Human  Anatomy.) 


-(Siibin 


follicle  and  form  with  it  a  simple  lymph  node  (Fig.  168).  This 
condition  is,  however,  in  many  cases  but  transitory,  the  artery 
branching  and  collections  of  lymphoid  tissue  forming  around  each 
of  the  branches,  so  that  a  series  of  follicles  is  formed,  which, 
together  with  the  surrounding  lymphatic  vessels,  becomes  enclosed 
by  a  connective- tissue  capsule  to  form  a  compound  lymph  node. 
Later  trabeculae  of  connective  tissue  extend  from    the  capsule 


DEVELOPMENT    OF    THE    SPLEEN  275 

toward  the  center  of  the  node,  between  the  folHcles,  the  lymphatic 
network  gives  rise  to  peripheral  and  central  lymph  sinuses,  and- 
the  follicles,  each  with  its  arterial  branch,  constitute  the  peripheral 
nodules  and  the  medullary  cords,  the  portions  of  these  immediately 
surrounding  the  leash  of  capillaries  into  which  the  artery  dissolves, 
constituting  the  so-called  germ  centers  in  which  multiplication 
of  the  lymphocytes  occurs. 

In  various  portions  of  the  body,  but  especially  along  the  root  of 
the  mesentery,  what  are  termed  hcemolymph  nodes  occur.  In 
these  the  lymph  sinus  is  replaced  by  a  blood  sinus,  but  with  this  ex- 
ception their  structure  resembles  that  of  an  ordinary  lymph  node, 
a  simple  one  consisting  of  a  follicle,  composed  of  adenoid  tissue 
with  a  central  blood-vessel,  and  a  peripheral  blood  sinus  (Fig. 
169). 

The  Development  of  the  Spleen.— Recent  studies  (Mall)  have 
shown  that  the  spleen  may  well  be  regarded  as  possessing  a  struc- 
ture comparable  to  that  of  the  lymph  nodes,  the  pulp  being  more  or 
less  distinctly  divided  by  trabeculae  into  areas  termed  pulp  cords, 
the  axis  of  each  of  which  is  occupied  by  a  twig  of  the  splenic  artery, 
while  the  Malpighian  corpuscles  may  be  regarded  as  lymph  follicles. 
The  spleen,  therefore,  seems  to  fall  into  the  same  category  of  or- 
gans as  the  lymph  and  haemolymph  nodes,  differing  from  these 
chiefly  in  the  absence  of  sinuses.  It  has  generally  been  regarded 
as  a  development  of  the  mesenchyme  situated  between  the  two 
layers  of  the  mesogastrium.  To  this  view,  however,  recent  ob- 
servers have  taken  exception,  holding  that  the  ultimate  origin  of 
the  organ  is  in  part  or  entirely  from  the  coelomic  epithelium  of  the 
left  layer  of  the  mesogastrium.  The  first  indication  of  the  spleen 
has  been  observed  in  embryos  of  the  fifth  week  as  a  sHght  elevation 
on  the  left  (dorsal)  surface  of  the  mesogastrium,  due  to  a  local 
thickening  and  vascularization  of  the  mesenchyme,  accompanied 
by  a  thickening  of  the  coelomic  epithelium  which  covers  the  ele- 
vation. The  mesenchyme  thickening  presents  no  differences  from 
the  neighboring  mesenchyme,  but  the  epithelium  is  not  distinctly 
separated  from  it  over  its  entire  surface,  as  it  is  elsewhere  in  the 
mesentery.     In  later  stages,  which  have  been  observed  in  detail 


276  DEVELOPMENT  OF  THE  SPLEEN 

in  pig  and  other  amnio te  embryos,  cells  separate  from  the  deeper 
layers  of  the  epithelium  (Fig.  1 70)  and  pass  into  the  mesenchyme 
thickening,  whose  tissue  soon  assumes  a  different  appearance  from 
the  surrounding  mesenchyme  by  its  cells  being  much  crowded. 
This  migration  soon  ceases,  however,  and  in  embryos  of  forty- two 
days  the  coelomic  epitheHum  covering  the  thickening  is  ^reduced  to 
a  simple  layer  of  cells. 

The  later  stages  of  development  consist  of  an  enlargement  of 
the  thickening  and  its  gradual  constriction  from  the  surface  of  the 


W 


Fig.  170. — Section  through  the  Left  Layer  of  the  Mesogastrium  of  a  Chick 

Embryo  of   Ninety-three  Hours,  Showing  the  Origin  of  the  Spleen, 

ep,  Coelomic  epithelium;  ms,  mesenchyme. — (Tonkoff.) 

mesogastrium,  until  it  is  finally  united  to  it  only  by  a  narrow  band 
through  which  the  large  splenic  vessels  gain  access  to  the  organ. 
The  cells  differentiate  themselves  into  trabeculae  and  pulp  cords 
special  collections  of  lymphoid  cells  around  the  branches  of  the 
splenic  artery  forming  the  Malphigian  corpuscles. 

It  has  already  been  pointed  out  (p.  227)  that  during  embryonic  life 
the  spleen  is  an  important  haematopoietic  organ,  both  red  and  white 
corpuscles  undergoing  active  formation  within  its  substance.  The 
Malpighian  corpuscles  are  collections  of  lymphocytes  in  which  multipli- 
cation takes  place,  and  while  nothing  is  as  yet  known  as  to  the  fate  of  the 
cells  which  are  contributed  to  the  spleen  from  the  coelomic  epithelium, 
since  they  quickly  come  to  resemble  the  mesenchyme  cells  with  which 
they  are  associated,  yet  the  growing  number  of  observations  indicating 
an  epithelial  origin  for  lymphocytes  suggests  the  possibility  that  the 
cells  in  question  may  be  responsible  for  the  first  leukocytes  of  the  spleen. 

The  Coccygeal  or  Luschka's  Ganglion. — In  embryos  of  about  15 
cm.  there  is  to  be  found  on  the  ventral  surface  of  the  apex  of  the 


LITERATURE  277 

coccyx  a  small  oval  group  of  polygonal  cells,  clearly  separated 
from  the  surrounding  tissue  by  a  mesenchymal  capsule.  Later- 
connective- tissue  trabecular  make  their  way  into  the  mass,  which 
thus  becomes  divided  into  lobules,  and,  at  the  same  time,  a  rich 
vascular  supply,  derived  principally  from  brandies  of  the  middle 
sacral  artery,  penetrates  the  body,  which  thus  assumes  the  adult 
condition  in  which  it  presents  a  general  resemblance  to  a  group  of 
lymph  follicles. 

It  has  generally  been  supposed  that  the  coccygeal  ganglion  was 
in  part  derived  from  the  sympathetic  nervous  system  and  belonged 
to  the  same  group  of  organs  as  the  suprarenal  bodies.  The  most 
recent  work  on  its  development  (Stoerk)  tends,  however,  to  dis- 
prove this  view,  and  the  ganglion  seems  accordingly  to  find  its 
place  among  the  lymphoid  organs. 

LITERATURE 

W.  A.  Baetjer:  *0n  the  Origin  of  the  Mesenteric   Sac  and  the  Thoracic  Duct 

in  the  Embryo  Pig,  '  Amer.  Journ,  Anat.,  l,  1908. 
E.  VAN  Beneden  and  C.  Julin:  "Recherches  sur  la  formation  des  annexes  foetales 

chez  les  mammiferes,"  Archiv.  de  Biolog.,  v,   1884. 
A.  C.  Bernays:  "  Entwicklungsgeschichte  der  Atrioventricularklappen,"  Morphol. 

Jahrbuch,  11,  1876. 
G.  Born:  "Beitrage  zur  Entwicklungsgeschichte  des  Saugethierherzens,"  Archif 

fiir  mikrosk.  Anat.,  xxxiii,  1889. 
J.  L.  Bremer:  "On  the  Origin  of  the  Pulmonary  Arteries  in  Mammals,"  Anat 

Record,  iii,  1909. 
I.  Broman:  "Ueber  die  Entwicklung,  Wanderung  und  Variation  der  Bauchaorten- 

zweige  bei  den  Wirbeltiere,"  Ergeb.  Anat.  und  Entwick.,xvi,  1906. 
I.  Broman:  "Ueber  die  Entwicklung  und  "Wanderung"  der  Zweige  der  aorta  ab- 

dominalis  beim  Menschen,"  Anat.  Hefte,xxxyi,  1908. 
E.  E.  Butterfield:  "Ueber  die  ungranulierte  Vorstufen  der  Myelocyten  und  ihre 

Bildung  in  Milz,  Leber  und  Lymphdriisen,"  Deutsch.  Arch.  J.  klin.  Med.,  xcii, 

1908. 
E.  R.  Clark:  "Observations  on  Living  Growing  Lymphatics  in  the  Tail  of  the  Frog 

Larva,"  Anat.  Record,  in,  1909. 

C.  B.  Coulter:  "The  Early  Development  of  the  Aortic  Arches  of  the  Cat,  with 

Especial  Reference  to  the  Presence  of  a  Fifth  Arch,"  Anat.  Record,  ni,  1909. 
Vera  Danchakoff:  "Origin  of  the  blood  cells.     Development  of  the  haematopoi- 
etic organs  and  regeneration  of  blood  cells  from  the  standpoint  of  the  monophy- 
letic  school,"  Anat.  Rec.,x,  1916. 

D.  M.  Davis:  "Studies  on  the  chief  veins  in  early  pig  embryos  and  the  origin  of  the 

vena  cava  inferior,"  Amer.  Journ.  Anat.,  x,  1910. 


278  LITERATURE 

J.  Disse:  "Die  Entstehung  des  Blutes  und  der  ersten  Gefasse  im  Huhnerei,"  Archiv 

fur  mikrosk.  Anat.,  xvi,  1879. 
A.  C.  F.  Eternod:  "Premiers  stades  de  la  circulation  sanguine  dans  I'ceuf  et  I'em- 

bryon  humain,"  Anat.  Anzeiger,  xv,  1899. 
H.  M.  Evans:  "On  the  Development  of  the  Aortae,  Cardinal  and  Umbilical  Veins, 

and  the  other  Blood-vessels  of  Vertebrate  Embryos  from  Capillaries,"  Anat. 

Record,  iii,  1909. 
V.  Federow:  "Ueber  die  Entwicklung  der  Lungenvene,"  Anat..  Hefte,  xl,  1910. 
W.    Felix:  "Zur    Entwicklungsgeschichte   der    Rumpfarterien   des   menschlichen 

Embryo,"  Morphol.  Jahrh.,  xli,  1910. 
G.  J.  Heuer:  "The  Development  of  the  Lymphatics  in  the  Small  Intestine  of  the 

Pig,"  Amer.  Journ.  Anat.,  rx,  1909. 
W.  His:  "Anatomic  menschlicher  Embryonen," Leipzig,  1880-1882. 
F.  Hochstetter:  "Ueber  die  ursprungliche  Hauptschlagader  der  hinteren  Glied- 

masse  des  Menschen  und  der  Saugethiere,  nebst  Bemerkungen  iiber  die  Ent- 
wicklung der  Endaste  der  Aorta  abdominalis,"  Morphol.  J ahrbuch,xvi,  1890. 
F.  Hochstetter:  "Ueber  die  Entwicklung  der  A.  vertebralis  beim  Kaninchen,  nebst 

Bemerkungen  iiber  die  Entstehung  der  Ansa  Vieusseni,"  Morphol.  Jahrhuch, 

XVI,  1890. 

F.  Hochstetter:  "Beitrage  zur  Entwicklungsgeschichte  des  Venensystems    der 

Amnioten,"  Morphol.  Jahrhuch,  xx,  1893. 
W.  H.  Howell:  " The  Life-history  of  the  Formed  Elements  of  the  Blood,  Especially 

the  Red  Blood-corpuscles,"  /owm.  of  Morphol.,  rv,  1890. 
W.  H.  Howell:  "Observations  on  the  Occurrence,  Structure,  and  Function  of  the 

Giant-cells  of  the  Marrow,"  Journ.  of  Morph.,  iv,  1890. 

G.  S.  Huntington:  "The  Genetic  Principles  of  the  Development  of  the  Systemic 

Lymphatic  Vessels  in  the  Mammalian  Embryo,"  Anat.  Record,  rv,  1910. 
G.  S.  Huntington:  "The  Anatomy  and  Development  of  the  Systemic  Lymphatic 

Vessels  of  the  Domestic  Cat,"  Memoirs  of  Wistar  Institute,  i,  191 2. 
G.  S.  Huntington:  "The  Development  of  the  Mammalian  Jugular  Lymph   Sac, 

Etc.,"  Amer.  Journ.  Anat.,  xvi,  1914. 
G.  S.  Huntington.     The  Morphology  of  the  Pulmonary  Artery  in  the  Mammalia," 

Anat.  Record,    vii,  19 19. 
G.  S.  Huntington  and  C.  F.  W.  McClure:  "Development  of  Post-cava  and  Tribu- 
taries in  the  Domestic  Cat,"  Amer.  Journ.  Anat.,  vi,  1907. 
G.  S.  Huntington  and  C.  F.  W.  McClure:  "The  Development  of  the  Main  Lymph 

Channels  of  the  Cat  in  their  Relations  to  the  Venous  System,"  Amer.  Journ. 

Anat.,  VI,  1907. 
G.  S.  Huntington  and  C.  F.  W.  McClure:  "The  Anatomy  and  Development  of 

the  Jugular  Lymph  Sacs  in  the  Domestic  Cat,"  Amer.  Journ.  Anat.,  x,  1910. 
H.  E.  Jordan:  "A  Microscopical  Study  of   the  Umbilical  Vesicle  of  a  13  mm. 

Human  Embryo,  with  Special  Reference  to  the  Entodermal  Tubules  and  the 

Blood  Islands,"  Anat.  Anzeiger,  xxxvii,  1910. 
O.  F.  Kampmeier:  "The  development  of  the  thoracic  duct  in  the  pig,"  Amer.  Journ. 

Anat.,  XIII,  191 2. 
C.  A.  Kling:  "Studien  uber  die  Entwicklung  der  Lymphdriisen  beim  Menschen," 

Archiv.  fiir  mikrosk.  Anat.,  lxiii,  1904. 


LITERATURE  279 

H.  Lehmann:  "On  the  Embryonic  History  of  the  Aortic  Arches  in  Mammals,"  Anat. 

Anzeiger,  xxvt,  1905.  _ 

F.  T.  Lewis:  ''The  Development  of  the  Vena  Cava  Inferior,"  Amer.  Journ.  of  Anat., 

I,  1902. 
F.  T,  Lewis:  "The  Development  of  the  Veins  in  the  Limbs  of  Rabbit  Embryos," 

Amer.  Journ.  Anat.,  v,  1906. 
F.T.  Lewis:  "The  Development  of  the  Lymphatic  System  in  Rabbits,"  Amer.  Journ. 

Anat.,  V,  1906. 
F.  T.  Lewis:  "On  the  Cervical  Veins  and  Lymphatics  in  Four  Human  Embryos," 

Amer.  Journ.  Anat.,  rx,  1909. 
F.  T.  Lewis:  "The  First  Lymph  Glands  in  Rabbit  and  Human  Embryos,"  Anat. 

Record,  in,  1909. 
W.  A.  Locy:  "The  Fifth  and  Sixth  Aortic  Arches  in  Chick  Embryos,  with  Comments 

on  the  Condition  of  the  same  Vessels  in  other  Vertebrates,"  Anat.  Anzeiger 

xxrx,  1906. 
F.  P.  Mall:  "Development  of  the  Internal  Mammary  and  Deep  Epigastric  Arteries 

in  Man,"  Johns  Hopkins  Hospital  Bulletin,  1898. 
F. P.  Mall:  "On  the  Development  of  the  Blood-vessels  of  the  Brain  in  the  Human 

Embryo,"  Amer.  Journ.  Anat.,  rv,  1905. 
F.  P.  Mall:  "On  the  Development  of  the  Human  Heart,"  Amer.  Journ.  Anat., 

XIII,  1912. 
A.  Maximow:  " Untersuchungen  iiber  Blut   und   Bindegewebe,"  Arch,    fur  mikr. 

Anat.,  Lxxiii,  1909;  lxxiv,  1909;  lxxvi,  19 id. 
C.F.W.  McClure:  "The  Development  of  the  Thoracic  and  Right  Lymphatic  Ducts 

in  the  Domestic  Cat  (Felis  Domestica),"  Anat.  Anzeiger,  xxxii,  1908. 
C.  F.  W.  McClure:  "The  Extra-intimal  Theory  of  the  Development  of  the  Mesen- 
teric   Lymphatics    in  the  Domestic  Cat,"    Verhandl.  Anat.  Gesellsch.,  xxiv, 

1910. 
C.  F.  W.  McClure:  "The  Development  of  the  Lymphatic  System  in  the  Light  of 

the  More  Recent  Investigations  in  the  Field  of  Vasculogenesis,"  Anat.  Rec, 

IX,  1915. 
C.  S.  Minot:  '^On  a  Hitherto  Unrecognized  Form  of  Blood  Circulation  without 

Capillaries  in  the  Organs  of  Vertebrata,"  Froc.  Boston  Soc.  Nat.  Hist.,  xxix,  1900. 
S.  Mollier:  "Die  Blutbildung  in  der  Embryonalen  Leber  des  Menschen  und  der 

Saiigetiere,"  Arch,  fur  mikrosk.  Anat.,  lxxiv,  1909. 
C.   V.  Morill:  "On  the  Development  of  the  Atrial  Septum  and  the  Valvular 

Apparatus  in  the  Right  Atrium  of  the  Pig  Embryo,"  Amer.  Journ.  Anat.,  xx, 

1916. 
A.  G.  Pohlman:  "The  Course  of  the  Blood  through  the  Fetal  Mammalian  Heart," 

Anat.  Record,  11,  1908. 
F.  Reagan:  "The  Fifth  Aortic  Arch  of  Mammalian  Embryos,"  Amer.  Journ.  Anat., 

XII,  1912. 
F.  P.  Reagan:  "Experimental  Studies  on  the  Origin  of  Vascular  Endothelium  and 

of  Erythrocytes,"  Amer.  Journ.  Anat.,  xxi,  191 7. 
E.  Retterer:  "Sur  la  part  que  prend  I'epithelium  a  la  formation  de  la  bourse  de 

Fabricius,  des  amygdales  et  des  plaques  de  Peyer,"  Journ.  de  I' Anat.  et  de  la 

Physiol.,  XXIX,  1893. 


28o  LITERATURE 

R.  Retzer:  "Some  Results  of  Recent  Investigations  on  the  Mammalian  Heart," 

Anat.  Record^  ii,  1908. 
C.  Rose:  "Zur  Entwicklungsgeschichte  des  Saugethierherzens,"  Morphol.  Jahrbuch, 

XV,  1889. 
Florence  R.  Sabin:  "On  the  Origin  of  the  Lymphatic  System  from  the  Veins  and 

the  Development  of  the  Lymph  Hearts  and  Thoracic  Duct  in  the  Pig,"  Amer. 

Journ.  of  Anat.,  i,  1902. 
Florence  R.  Sabin:  "The  Development  of  the  Lymphatic  Nodes  in  the  Pig  and 

their  Relation  to  the  Lymph  Hearts,"  Amer.  Journ.  Anat.,  rv,  1905. 
Florence  R.  Sabin:  "Further  Evidence  on  the  Origin  of  the  Lymphatic  Endothe- 
lium from  the  Endothelium  of  the  Blood  Vascular  System,"  Anat.  Record,  11, 

1908. 
Florence  R.  Sabin:  "On  the  Development  of  the  Lymphatic  System  in  Human 

Embryos  with  a  Consideration  of  the  Morphology  of  the  System  as  a  Whole/' 

Amer.  Journ.  Anat.,  rx,  1909. 
Florence  R.  Sabin:  "A  Critical  Study  of  the  Evidence  Presented  in  Several  Recent 

Articles  on  the  Development  of  the  Lymphatic  System,"  Anat.  Record,  v, 

1911. 
Florence  R.   Sabin:  "Der  Ursprung  und  die  Entwicklung  des  Lymphgefass- 

systems,"  Ergh.  Anat.  u.  Entw.,  xxi,  1913. 
Florence  R.  Sabin:  "On  the  Fate  of  the  Posterior  Cardinal  Veins,  etc.,  in  the 

Embryo  Pig,"  Carnegie  Inst.  Puh.  Contrih.  to  EmbryoL,  in,  1915. 
Florence  R.  Sabin:  "Preliminary  Note  on  the  Differentiation  of  Angioblasts 

and  the  Method  by  which  they  Produce  Blood-vessels,  Blood  plasma  and  Red 

Blood-cells  as  Seen  in  the  Living  Chick,"  Anat.  Rec,  xiii,  191 7. 
F.  Saxer:  "Ueber  die  Entwicklung  und  der  Bau  normaler  Lymphdriisen  und  die 

Entstehung  der  roten  und  weissen  BlutkSrperchen,"  Anat.  Hefte,  vi,  1896. 
R.  E.  ScAMMON  AND  E.  H.  NoRRis:  "On  the  Time  of  the  Post-natal  Obliteration  of 

the  Fetal  Blood-passages  (Foramen  Ovale,  Ductus  Arteriosus,  Ductus  Venosus), 

Anat.  Rec.  xv,  191 8. 
H.  Schridde:  "Die  Entstehung  der  ersten  embryonalen  Blutzellen  des  Menschen," 

Folia  hoematol,  iv,  1907. 
H.  VON  W.  Schulte:  "Early  Stages  of  Vasculogenesis  in  the  Cat  (Felis  Domestics), 

with  Especial  Reference  to  the  Mesenchymal  Origin  of   Endothelium,  Mem. 

Wistar  Inst.,  No.  3,  1914. 
H.  VON  W.  Schulte:  "The  Fusion  of  the  Cardiac  Anlages  and  the  Formation  of  the 

Cardiac  Loop  in  the  Cat  (Felis  Domestica),"  Amer.  Journ.  Anat.,  xx,  1916. 
H.  D.  Senior:  "The  Development  of  the  Arteries  of  the  Human  Lower  Extremity, 

Amer.  Journ.  Anal ,  xxv,  1919.     See  also  Anat.  Record,  xvii,  1920. 
H.  D.  Senior:  "An  Interpretation  of  the  Recorded  Arterial  Anomalies  of  the 

Human  Leg  and  Foot,"  Journ.  Anat.,  liii,  1919. 
L.  Stienon:  "Sur  la  Fermature  du  Canal  de  Botal,"  Arch,  de  Biol.,  xxvii,  1912. 
C.  R.  Stockard:  "The  Origin  of  Blood  and  Vascular  Endothelium  in  Embryos 

without  a  Circulation  of  the  Blood  and  in  the  .Normal  Embryo,"  Amer.  Journ. 

Anat.,  xviii,  1915. 
P.  Stohr:  "Ueber  die  Entwicklung  der  Darmlymphknotchen  und  uber  die  Riick- 

bildung  von  Darmdriisen."     Archiv  fiir  mikrosk.  Anat.,  li,  1898. 


LITERATURE  28 1 

O.  Stoerk:  "Ueber  die  Chromreaktion  der  Glandula  coccygea  lind  die  Beziehung, 

dieser  Driise  zum  Nervus  sympathicus,"  Arch,  fur  mikroskop.  Anai.,  Lxrx,  1906. 
G.  L.  Streeter:  "The  Development  of  the  Venous  Sinuses  of  the  Dura  Mater  in 

the  Human  Embryo,"  Amer.  Journ.  Anat.,xvin,  1915. 
G.  L.  Streeter:  "The  Developmental  Alterations  in  the  Vascular  System  of  the 

Brain  of  the  Human  Embryo,"  Carnegie  Inst.  Publ.  271,  Contrih.  to  Embryol. 

No.  24,  1919. 
O.  VAN  DER  Stricht:  "Nouvelles  recherches  sur  la  genese  des  globules  rouges  et  des 

globules  blancs  du  sang,"  Archives  de  Biolog.,  xii,  1892. 
O.  VAN  DER  Stricht:  " De  la  premiere  origine  du  sang  et  des  capillaires  sanguins  dans 

I'aire  vasculaire  du  Lapin,"  Comptes  Rendus  de  la  Soc.  de  Biolog.  Paris,  S6r.  10, 

II,  1895. 
J.  Tandler:  "Zur  Entwicklungsgeschichte  der  Kopfarterien  bei  den  Mammalia," 

Morphol.  Jahrhuch,  xxx,  1902. 
J.  Tandler:  "Zur  Entwickelungsgeschichte  der  menschlichen  Darmarterien,"  Anat. 

Hefie,  XKiii,  igo^. 
J.  Tandler:  "Ueber  die  Varietaten  der  arteria  coeliaca  und  deren  Entwicklung," 

Anat.  Hefte,  xxv,  1904. 
J.  Tandler:  "Ueber  die  Entwicklung  des  fiinften  Aortenbogens  und  der  fiinften 

Schlundtasche  beim  Menschen,"  Anat.  Hefte,  xxxviii,  1909. 
W.  Tonkoff:  "Die  Entwickelung  der  Milz  bei  den  Amnioten,"  Arch.  fUr  mikrosk. 

Anat.,  lvi,  1900. 
Bertha  de  Vriese:  "Recherches  sur  revolution  des  vaissaux  sanguins  des  membres 

chez  I'homme,"  Archiv  de  Biolog.,  xvin,  1902. 
F.  Weidenreich:  "Die  roten  Blutkorperchen,"  Ergb.  Anat.  und Entwick., xm,  1903, 

xrv,  1904. 
F.  Weidenreich:  "Die  Leucocyten  und  verwandte  Zellformen,"  Ergeb.  Anat.  und 

Entwick.,  XVI,  191 1. 
J.  H.  Wright:  "The  Histogenesis  of  the  Blood  Platelets,"  Journ.  of  Morph.,  xxi, 

1910, 


CHAPTER  X 

THE  DEVELOPMENT  OF  THE  DIGESTIVE  TRACT  AND 

GLANDS 

The  greatest  portion  of  the  digestive  tract  is  formed  by  the 
constriction  off  of  the  dorsal  portion  of  the  yolk-sac,  as  shown  in 
Fig.  53,  the  result  being  the  formation  of  a  cylinder,  closed  at  either 
end  and  composed  of  a  layer  of  splanchnic  mesoderm  lined  on 
its  inner  surface  by  endoderm.  This  cylinder  is  termed  the  archen- 
teron  and  has  connected  with  it  the  yolk-stalk  and  the  allantois, 
the  latter  communicating  with  its  somewhat  dilated  terminal 
portion,  which  also  receives  the  ducts  of  the  primitive  kidneys 
and  is  known  as  the  cloaca  (Fig.  172). 

At  a  very  early  stage  of  development  the  anterior  end  of  the 
embryo  begins  to  project  slightly  in  front  of  the  yolk-sac,  so  that  a 
shallow  depression  is  formed  between  the  two  structures.  As  the 
constriction  of  the  embryo  from  the  sac  proceeds,  the  anterior 
portion  of  the  brain  becomes  bent  ventrally  and  the  heart  makes 
its  appearance  immediately  in  front  of  the  anterior  surface  of  the 
yolk-sac,  and  so  the  depression  mentioned  above  becomes  deep- 
ened (Fig.  171)  to  form  the  oral  sinus.  The  floor  of  this,  lined  by 
ectoderm,  is  immediately  opposite  the  anterior  end  of  the  archen- 
teron,  and,  since  mesoderm  does  not  develop  in  this  region,  the 
ectoderm  of  the  sinus  and  the  endoderm  of  the  archenteron  are 
directly  in  contact,  forming  a  thin  pharyngeal  membrane  separating 
the  two  cavities  (Fig.  171,  pm).  In  embryos  of  2.15  mm.  this 
membrane  is  still  existent,  but  soon  after  it  becomes  perforated 
and  finally  disappears,  so  that  the  archenteron  and  oral  sinus 
become  continuous. 

Toward  its  posterior  end  the  archenteron  comes  into  somewhat 
similar  relations  with  the  ectoderm,  though  a  marked  difference  is 

282 


DEVELOPMENT    OF    THE    DIGESTIVE    TRACT 


283 


noticeable  in  that  the  area  over  which  the  cloacal  endoderm  is  in 
contact  with  the  ectoderm  to  form  the  cloacal  membrane  (Figr 
172,  cm)  lies  a  little  in  front  of  the  actual  end  of  the  archenteric 
cylinder,  the  portion  of  the  latter  which  lies  posterior  to  the  mem- 
brane forming  what  has  been  termed  the  postanal  gut  {p. an). 
This  diminishes  in  size  during  development  and  early  disappears 
altogether,  and  the  pouch-like  fold  seen  in  Fig.  172  between  the 
intestinal  portion  of  the  archeiiteron  and  the  allantoic  stalk  {at) 
deepening  until  its  floor  comes 
into  contact  with  the  cloacal 
membrane,  the  cloaca  becomes 
divided  into  a  ventral  portion, 
with  which  the  allantois  and 
the  primitive  excretory  ducts 
{w)  are  connected,  and  a  dorsal 
portion  which  becomes  the  lower 
end  of  the  rectum.  This  latter 
abuts  upon  the  dorsal  portion 
of  the  cloacal  membrane,  and 
this  eventually  ruptures,  so 
that  the  posterior  communi- 
cation of  the  archenteron  with 
the  exterior  becomes  estab- 
lished. This  rupture,  however, 
does  not  occur  until  a  com- 
paratively late  period  of  development,  until  after  the  embryo  has 
reached  the  fetal  stage;  nor  does  the  position  of  the  membrane 
correspond  with  the  adult  anus,  since  later  there  is  a  considerable 
development  of  mesoderm  around  the  mouth  of  the  cloaca,  bulg- 
ing out,  as  it  were,  the  surrounding  ectoderm,  more  especially 
anteriorly  where  it  forms  the  large  genital  tubercle  (see  Chapter 
XIII),  and  posteriorly  where  it  produces  the  anal  tubercle.  This 
appears  as  a  rounded  elevation  on  each  side  of  the  median  line, 
immediately  behind  the  cloacal  membrane  and  separated  from  the 
root. of  the  caudal  projection  by  a  depression,  the  precaudal  recess. 
Later  the  two  elevations  unite  across  the  median  line  to  form  a 


Fig.  171. — Reconstruction  of  the 
Anterior  Portion  of  an  Embryo  of 

2.15  MM. 

ah.  Aortic  bulb;  h,  heart;  o,  auditory 
capsule;  op,  optic  evagination;  pm, 
pharyngeal  membrane. — (His.) 


\7^ 


284 


DIGESTIVE    TRACT    AND    GLANDS 


transverse  ridge,  the  ends  of  which  curve  forward  and  eventually 
meet  in  front  of  the  original  and  orifice.  From  the  mesoderm  of 
the  circular  elevation  thus  produced  the  external  sphincter  ani 
muscle  is  formed,  and  it  would  seem  that  so  much  of  the  lower  end 
of  the  rectum  as  corresponds  to  this  muscle  is  formed  by  the  inner 
surface  of  the  elevation  and  is  therefore  ectodermal.  The  definite 
anus  being  at  the  end  of  this  terminal  portion  of  the  gut  is  there- 
fore some  distance  away  from  the  position  of  the  original  cloacal 
membrane. 


-nc 


Fig.  172. — Reconstruction  of  the  Hind  End  of  an  Embryo  6.5  mm.  Long. 
al,  Allantois;  &,  belly-stalk;  cl,  cloaca;  cm,  cloacal  membrane;  i,  intestine;  n. 
spinal  cord;  nc,  notochord;  p.an,  postanal  gut;  ur,  outgrowth  to  form; ureter  and 
metanephros;  w.  Wolffian  duct. — (Keibel.) 


It  will  be  noticed  that  the  digestive  tract  thus  formed  consists 
of  three  distinct  portions,  an  anterior,  short,  ectodermal  portion, 
an  endodermal  portion  representing  the  original  archenteron,  and 
a  posterior  short  portion  which  is  also  ectodermal.  The  differen- 
tiation of  the  tract  into  its  various  regions  and  the  formation  of 
the  various  organs  found  in  relation  with  these  may  now  be  con- 
sidered. 


DEVELOPMENT  OF  THE  MOUTH  REGION         285 

The  Development  of  the  Mouth  Region. — The  deepening  of 
the  oral  sinus  by  the  development  of  the  first  branchial  arch  an^ 
its  separation  into  the  oral  and  nasal  cavities  by  the  development 
of  the  palate  have  already  been  described  (p.  102),  but,  for  the 
sake  of  continuity  in  description,  the  latter  process  may  be  briefly 
recalled.  At  first  the  nasal  pits  communicate  with  the  oral  sinus 
by  grooves  lying  one  on  each  side  of  the  fronto-nasal  process,  but 
by  the  union  of  the  latter,  through  its  processus  globulares,  with 
the  maxillary  processes  these  communications  are  interrupted  and 
the  floors  of  the  nasal  pits  are  separated  from  the  oral  cavity  by 
thin  hucco-nasal  membranes,  formed  of  the  nasal  epithelium  in 
contact  with  that  of  the  oral  cavity.  In  embryos  of  about  15  mm. 
these  membranes  break  through  and  disappear,  and  the  nasal  and 
oral  cavities  are  again  in  communication,  but  the  communications 
are  now  behind  the  maxillary  processes  and  constitute  what  are 
termed  the  primitive  choance.  The  oral  cavity  at  this  stage  does 
not,  however,  correspond  with  the  adult  mouth  cavity,  since  there 
is  as  yet  no  palate,  the  roof  of  the  oral  cavity  being  the  base  of 
the  skull.  From  the  maxillopalatine  portions  of  the  upper  jaw, 
shelf-like  ridges  begin  to  grow,  being  at  first  directed  downward 
so  that  their  surfaces  are  parallel  with  the  sides  of  the  tongue, 
which  projects  up  between  them.  Later,  however,  they  become 
bent  upward  to  a  horizontal  position  (Fig.  173)  and  eventually 
meet  in  the  median  line  to  form  the  palate,  separating  the  nasal 
cavities  from  the  mouth  cavity.  All  that  portion  of  the  original 
oral  cavity  which  Hes  behind  the  posterior  edge  of  the  palatal 
shelf  is  now  known  as  the  pharynx,  the  boundary  between  this 
and  the  mouth  cavity  being  emphasized  by  the  prolongation 
backward  and  downward  of  the  posterior  angles  of  the  palatal  shelf 
as  ridges,  which  form  the  pharyngo-palatine  arches  (posterior 
pillars  of  the  fauces) .  The  nasal  cavities  now  communicate  with 
the  upper  part  of  the  pharynx  (naso-pharynx)  by  the  posterior 
choanae.  The  palatal  processes  are  entirely  derived  from  the 
maxillary  processes,  the  premaxillary  portion  of  the  upper  jaw 
which  is  a  derivative  of  the  fronto-nasal  process,  not  taking  part 
in  their  formation.     Consequently  a  gap  exists  between  the  palatal 


286         DEVELOPMENT  OF  THE  MOUTH  REGION 

shelves  and  the  premaxillae  for  a  time,  by  which  the  nasal  and 
mouth  cavities  communicate;  it  places  the  organ  of  Jacobson 
(see  p.  434)  in  communication  with  the  mouth  cavity  and  may 
persist  until  after  birth.  Later  it  becomes  closed  over  by  mucous 
membrane,  but  may  be  recognized  in  the  dried  skull  as  the  fora- 
men incisivum  (anterior  palatine  canal). 

Occasionally  there  is  a  failure  of  the  union  of  the  palatal  plates,  the 
condition  known  as  cleft  palate  resulting.  The  inhibition  of  develop- 
ment which  brings  about  this  condition  may  take  place  at  different 
stages,  but  frequently  it  occurs  while  the  plates  still  have  an  almost 
vertical  direction.     Typically  cleft  palate  is  a  deficiency  in  the  median 


Fig.   173. — View  of  iiu:  Roof  of  the  Oral  Fossa  of  Embryo  showing  the  Lip- 
Groove  AND  the  Formation  of  the  Palate. — {His.) 

line  of  the  roof  of  the  mouth,  not  affecting  the  upper  jaw,  but  very 
frequently  it  is  combined  with  the  defect  which  produced  hare-lip 
(see  p.  98),  in  which  case  the  cleft  may  be  continued  through  the  upper 
jaw  between  its  maxillary  and  premaxillary  portions  on  either  or  both 
sides,  according  to  the  extent  of  the  defect. 

At  about  the  fifth  week  of  development  a  downgrowth  of  epi- 
thelium into  the  substance  of  both  the  maxillary  and  fronto-nasal 
processes  above  and  the  mandibular  process  below  takes  place, 
and  the  surface  of  the  downgrowth  becomes  marked  by  a  deepen- 
ing groove  (Fig.  173),  which  separates  an  anterior  fold,  the  lip, 
from  the  jaw  proper  (Fig.  174).  Mention  should  also  be  made  of 
the  fact  that  at  an  early  stage  of  development  a  pouch  is  formed 
in  the  median  line  of  the  roof  of  the  oral  sinus,  just  in  front  of  the 
pharyngeal  membrane,  by  an  outgrowth  of  the  epithelium.     This 


DEVELOPMENT    OF    THE    TEETH  287 

pouch,  known  as  Rathke^s  pouch,  comes  in  contact  above  with  a 
downgrowth  from  the  floor  of  the  brain  and  forms  with  it  the- 
pituitary  body  (see  p.  403). 

The  Development  of  the  Teeth. — When  the  epithelial  down- 
growth  which  gives  rise  to  the  lip  groove  is  formed,  a  horizontal 
outgrowth  develops  from  it  which  extends  backward  into  the  sub- 
stance of  the  jaw,  forming  what  is  termed  the  dental  shelf  (Fig.  174 
A).  This  at  first  is  situated  on  the  anterior  surface  of  the  jaw, 
but  with  the  continued  development  of  the  lip  fold  it  is  gradually 
shifted  until  it  comes  to  lie  upon  the  free  surface  (Fig.  174,5), 
where  its  superficial  edge  is  marked  by  a  distinct  groove,  the  dental 
groove  (Fig.  173).  At  first  the  dental  shelf  of  each  jaw  is  a  con- 
tinuous plate  of  cells,  uniform  in  thickness  throughout  its  entire 
width,  but  later  ten  thickenings  develop  upon  its  deep  edge,  and 
beneath  each  of  these  the  mesoderm  condenses  to  form  a  dental 
papilla,  over  the  surface  of  which  the  thickening  moulds  itself  to 
form  a  cap,  termed  the  enamel  organ  (Fig.  174,  B).  These  ten 
papillae  in  each  jaw,  with  their  enamel  caps,  represent  the  teeth 
of  the  first  dentition. 

The  papillae  do  not,  however,  project  into  the  very  edge  of  the 
dental  shelf,  but  obliquely  into  what,  in  the  lower  jaw,  was  origi- 
nally its  under  surface  (Fig.  174,  B),  so  that  the  edge  of  the  shelf  is 
free  to  grow  still  deeper  into  the  substance  of  the  jaw.  This  it 
does,  and  upon  the  extension  so  formed  there  is  developed  in  each 
jaw  a  second  set  of  thickenings,  beneath  each  of  which  a  dental 
papilla  again  appears.  These  tooth-germs  represent  the  incisors 
canines,  and  premolars  of  the  permanent  dentition.  The  lateral 
edges  of  the  dental  shelf  being  continued  outward  toward  the  arti- 
culations of  the  jaws  as  prolongations  which  are  not  connected 
with  the  surface  epithelium,  opportunity  is  afforded  for  the  develop- 
ment of  three  additional  thickenings  on  each  side  in  each  jaw,  and, 
papillae  developing  beneath  these,  twelve  additional  tooth-germs 
are  formed.  These  represent  the  permanent  molars;  their  forma- 
tion is  much  later  than  that  of  the  other  teeth,  the  germ  of ^the 
second  molar  not  appearing  until  about  the  sixth  week  after  birth, 
while  that  of  the  third  is  delayed  until  about  the  fifth  year. 


288  DEVELOPMENT    OF   THE    TEETH 

As  the  tooth-germs  increase  in  size,  they  approach  nearer  and 
nearer  to  the  surface  of  the  jaw,  and  at  the  same  time  the  enamel 
organs  separate  from  the  dental  shelf  until  their  connection  with  it 
is  a  mere  neck  of  epithelial  cells.  In  the  meantime  the  dental 
shelf  itself  has  been  undergoing  degeneration  and  is  reduced  to  a 
reticulum  which  eventually  completely  disappears,  though  frag- 
ments of  it  may  occasionally  persist  and  give  rise  to  various  mal- 
formations.    With  the  disappearance  of  the  last  remains  of  the 


Fig.  174. — Transverse  Sections  through  the  Lower  Jaw  showing  the 
Formation  of  the  Dental  Shelf  in  Embryos  of  (A)  17  mm.  and  (J3)  40  mm. — 
{Rose.) 


shelf,  the  various  tooth-germs  naturally  lose  all  connection  with 
one  another. 

It  will  be  seen,  from  what  has  been  said,  that  each  tooth-germ 
consists  of  two  portions,  one  of  which,  the  enamel  organ,  is  de- 
rived from  the  ectoderm,  while  the  other,  the  dental  papilla,  is 
mesenchymatous.  Each  of  these  gives  rise  to  a  definite  portion  of 
the  fully  formed  tooth,  the  enamel  organ,  as  its  name  indicates, 


DEVELOPMENT    OF    THE    TEETH 


289 


producing  the  enamel,  while  from  the  dental  papilla  the  dentine 
and  pulp  are  formed.  — 

The  cells  of  the  enamel  organ  which  are  in  contact  with  the  sur- 
face of  the  papilla,  at  an  early  stage  assume  a  cylindrical  form  and 


od. 


Fig.  175. — Section  through  the  First  Molar  Tooth  of  a  Rat,  Twelve  Days 

Old. 
Ap,  Periosteum;  R,  dentine;  Rp,  epidermis;  Od,  odontoblasts;  S,  enamel;  SRa 
and  SRi,  outer  and  inner  layers  of  the  enamel  organ;  SR,  portion  of  the  enamel 
organ  which  does  not  produce  enamel, — {von  Brunn.) 

become  arranged  in  a  definite  layer,  the  enamel  membrane  (Fig. 
175,  SEi),  while  the  remaining  cells  (SEa)  apparently  degenerate 
eventually,  though  they  persist  for  a  time  to  form  what  has  been 
termed  the  enamel  pulp.     The  formation  of  the  enamel  seems  to  be 

19 


290  DEVELOPMENT   OF   THE   TEETH 

due  to  the  direct  transformation  of  the  enamel  cells,  the  process  be- 
ginning at  the  basal  portion  of  each  cell,  and  as  a  result,  the  enamel 
consists  of  a  series  of  prisms,  each  of  which  represents  one  of  the 
cells  of  the  enamel  membrane.  The  transformation  proceeds 
until  the  cells  have  become  completely  converted  into  enamel 
prisms,  except  at  their  very  tips,  which  form  a  thin  membrane,  the 
enamel  cuticle,  which  is  shed  soon  after  the  eruption  of  the  teeth. 

The  dental  papillae  are  at  first  composed  of  a  closely  packed 
mass  of  mesenchyme  cells,  which  later  become  differentiated  into 
connective  tissue  into  which  blood-vessels  and  nerves  penetrate. 
The  superficial  cells  form  a  more  or  less  definite  layer  (Fig.  175,0^), 
and  are  termed  odontoblasts,  having  the  function  of  manufacturing 
the  dentine.  This  they  accomplish  in  the  same  manner  as  that  in 
which  the  periosteal  osteoblasts  produce  bone,  depositing  the  den- 
tine between  their  surfaces  and  the  adjacent  surface  of  the  enamel. 
The  outer  surface  of  each  odontoblast  is  drawn  out  into  a  number 
of  exceedingly  fine  processes  which  extend  into  the  dentine  to  oc- 
cupy the  minute  dentinal  tubules,  just  as  processes  of  the  osteo- 
blasts occupy  the  canaHculi  of  bone. 

At  an  early  stage  the  enamel  membrane  forms  an  almost  com- 
plete investment  for  the  dental  papilla  (Fig.  175),  but  as  the  ossifi- 
cation of  the  tooth  proceeds,  it  recedes  from  the  lower  part,  until 
finally  it  is  confined  entirely  to  the  crown.  The  dentine  forming 
the  roots  of  the  tooth  then  becomes  enclosed  in  a  layer  of  cement, 
which  is  true  bone  and  serves  to  unite  the  tooth  firmly  to  the  walls 
of  its  socket.  As  the  tooth  increases  in  size,  its  extremity  is 
brought  nearer  to  the  surface  of  the  gum  and  eventually  breaks 
through,  the  eruption  of  the  first  teeth  usually  taking  place  during 
the  last  half  of  the  first  year  after  birth.  The  growth  of  the  per- 
manent teeth  proceeds  slowly  at  first,  but  later  it  becomes  more 
rapid  and  produces  pressure  upon  the  roots  of  the  primary  teeth. 
These  roots  then  undergo  partial  absorption,  and  the  teeth  are 
thus  loosened  in  their  sockets  and  are  readily  pushed  out  by  the 
further  growth  of  the  permanent  teeth. 

The  dates  and  order  of  the  eruption  of  the  teeth  are  subject  to  con- 
siderable variation,  but  the  usual  sequence  is  somewhat  as  follows: 


DEVELOPMENT    OF   THE   TONGUE  29 T 

-    Primary  Dentition. 

Median  incisors 6th  to  8th  month.  

Lateral  incisors 8th  to  12th  month. 

First  molars Beginning  of  2d  year. 

Canines 1 3'^  years. 

Second  molars 3  to  3  3^  years. 

Permanent  Dentition 

First  molars 7th  year. 

Middle  incisors 8th  year. 

Lateral  incisors gth  year. 

First  premolars loth  year. 

Second  premolars nth  year. 

Canines            1  4.1,  4.        4.u 

Second  molars  I 13th  to  14*  years. 

Third  molars 1 7th  to  40th  years. 

In  a  considerable  percentage  of  individuals  the  third  molars  (wisdom 
teeth)  never  break  through  the  gums,  and  frequently  when  they  do  so 
they  fail  to  reach  the  level  of  the  other  teeth,  and  so  are  only  partly 
functional.  These  and  other  peculiarities  of  a  structural  nature 
shown  by  these  teeth  indicate  that  they  are  undergoing  a  retrogressive 
evolution. 

The  Development  of  the  Tongue. — Strictly  speaking,  the 
tongue  is  largely  a  development  of  the  pharyngeal  region  of  the 
digestive  tract  and  only  secondarily  grows  forward  into  the  floor 
of  the  mouth.  In  embryos  of  about  3  mm.  there  may  be  seen  in 
the  median  line  of  the  floor  of  the  mouth,  between  the  ventral  ends 
of  the  first  and  second  branchial  arches,  a  small  rounded  elevation 
which  has  been  termed  the  tuherculum  impar  (Fig.  176,  Ti).  It 
was  at  one  time  believed  that  this  gave  rise  to  the  anterior  portion 
of  the  tongue,  but  recent  observations  seem  to  show  that  it  reaches 
its  greatest  development  in  embryos  of  about  8  mm.,  after  which 
it  becomes  less  prominent  and  finally  unrecognizable.  But  before 
this  occurs  a  swelling  appears  in  the  anterior  part  of  the  mouth  on 
each  side  of  the  median  line  (Fig.  176,  /),  and  these  gradually 
increase  in  size  and  eventually  unite  in  the  median  line  to  form 
the  main  mass  of  the  body  of  the  tongue.  They  are  separated 
from  the  neighboring  portions  of  the  first  branchial  arch  by  a  deep 
groove,  the  alveolo-lingual  groove,  and  posteriorly  are  separated 


292 


DEVELOPMENT  OF  THE  TONGUE 


from  the  second  arch  by  a  groove  which  later  becomes  distinctly 
V-shaped  (Fig.  177),  a  deep  depression,  which  gives  rise  to  the 

r< 

i 


-Cop 


Fig.  176. — Floor  of  the  Mouth  and  Pharynx  of  an  Embryo  of  7.5  mm.,  from 

A  Reconstruction. 

Cop.  Copula;  /,  furcula;  t,  swelling  that  gives  rise  to  the  body  of  the  tongue;  Ti, 

tuberculum  impar;  I-III,  branchial  arches, 

thyreoid  body,  lying  at  the  apex  of  the  V.     Behind  the  thyreoid 
pouch  .the  ventral  ends  of  the  second  and  third  branchial  arches 

unite    to   form    an   elevation,    the 

copula  (Fig.   176,  Cop),  and  from 

^^        /  ^[^fcfer^f    this  and  the  adjacent  portions  of 

the  second  and  third  arches  the 
posterior  portion  of  the  tongue 
develops. 

The  tongue  then  consists  of  two 
distinct  portions,  which  eventually 
fuse  together,  but  the  groove  which 
Fig.  177.— The  Floor  of  the  originally  separated   them    remains 
Pharynx  of  an  Embryo  of  about  j^ore  or  less  clearly  distinguishable 

20  MM.  -^  *=» 

«^,  Epiglottis;  /c,  foramen  caecum; 
t^  and  f  2  median  and  lateral  portions 
of  the  tongue. — {His.) 


f( 


(Fig.  177),  the  vallate  papillae  (see 
p.  435)  developing  immediately  an- 
terior to  it. 


The  tongue  is  essentially  a  muscular  .organ,  being  formed  of  a 
central  mass  of  muscular  tissue,  enclosed  at  the  sides  and  dorsally  by 
mucous  membrane  derived  from  the  floor  of  the  mouth  and  pharynx. 


THE    SALIVARY   GLANDS  293 

The  muscular  tissue  consists  partly  of  fibers  limited  to  the  substance  of 
the  tongue  and  forming  the  m.  lingualis,  and  also  of  a  number  of  ex- 
trinsic muscles,  the  hyoglossi,  genioglossi,  styloglossi,  glossopalatini  and 
chondroglossi.  The  last  two  muscles  are  innervated  by  the  vagus 
nerve,  and  the  remaining  extrinsic  muscles  receive  fibers  from  the 
hypoglossal,  while  the  lingualis  is  supplied  partly  by  the  hypoglossal 
and  partly,  apparently,  by  the  facial  through  the  chorda  tympani. 
That  the  facial  should  take  part  in  the  supply  is  what  might  be  ex- 
pected from  the  mode  of  development  of  the  tongue,  but  the  hypo- 
glossal has  been  seen  to  correspond  to  certain  primarily  postcranial 
metameres  (p.  172),  and  its  relation  to  structures  taking  part  in  the 
formation  of  an  organ  belonging  to  the  anterior  part  of  the  pharynx 
seems  somewhat  anomalous.  It  may  be  supposed  that  in  the  evolu- 
tion of  the  tongue  the  extrinsic  muscles,  together  with  a  certain  amount 
of  the  lingualis,  have  grown  into  the  tongue  thickenings  from  regions 
situated  much  further  back,  for  the  most  part  from  behind  the  last 
branchial  arch. 

Such  an  invasion  of  the  tongue  by  muscles  from  posterior  segments 
would  explain  the  distribution  of  its  sensory  nerves  (Fig.  178).  The 
anterior  portion,  from  its  position,  would  naturally  be  supplied  by 
branches  from  the  fifth  and  seventh  nerves,  while  the  posterior  portion 
might  be  expected  to  be  supplied  by  the  seventh.  There  seems,  how- 
ever, to  have  been  a  dislocation  forward,  if  it  may  be  so  expressed,  of  the 
mucous  membrane,  the  sensory  distribution  of  the  ninth  nerve  extend- 
ing forward  upon  the  posterior  part  of  the  anterior  portion  of  the 
tongue,  while  a  considerable  amount  of  the  posterior  portion  is  supplied 
by  the  tenth  nerve.  The  distribution  of  the  sensory  fibers  of  the  facial 
is  probably  confined  entirely  to  the  anterior  portion,  though  further  in- 
formation is  needed  to  determine  the  exact  distribution  of  both  the 
motor  and  sensory  fibers  of  this  nerve  in  the  tongue. 

The  Development  of  the  Salivary  Glands. — In  en^bryos  of 
about  8  mra.  a  slight  furrow  may  be  observed  in  the  floor  of  the 
groove  which  connects  the  lip  grooves  of  the  upper  and  lower  jaws 
at  the  angle  of  the  mouth  and  may  be  known  as  the  cheek  groove. 
In  later  stages  this  furrow  deepens  and  eventually  becomes  closed 
in  to  form  a  hollow  tubular  structure,  which  in  embryos  of  17  mm. 
has  separated  from  the  epithelium  of  the  floor  of  .the  cheek  groove 
except  at  its  anterior  end  and  has  become  embedded  in  the  con- 
nective tissue  of  the  cheek.  This  tube  is  readily  recognizable  as 
the  parotid  gland  and  duct,  and  from  the  latter  as  it  passes  across 
the  masseter  muscle  a  pouch-like  outgrowth  is  early  formed  which 
probably  represents  the  socia  parotidis. 


294 


THE    SALIVARY    GLANDS 


The  submaxillary  gland  and  duct  appear  in  embryos  of  about 
13  mm.  as  a  longitudinal  ridge-like  thickening  of  the  epithelium 
of  the  floor  of  the  alveolo-lingual  groove  (see  p.  291).  This  ridge 
gradually  separates  from  behind  forward  from  the  floor  of  the 
groove  and  sinks  into  the  subjacent  connective  tissue,  retaining, 
however,  its  connection  with  the  epithelium  at  its  anterior  end. 


Fig.   1 78. — Diagram  of  the  Distribution  of  the  Sensory  nerves  of  the  Tongue. 
The  area  supplied  by  the  fifth  (and  seventh)  nerve  is  indicated  by  the  transverse 
lines;  that  of  the  ninth  by  the  oblique  lines;  and  that  of  the  tenth  by  the  small  circles. 
— (Zander.) 

which  indicates  the  position  of  the  opening  of  the  duct.  In  the 
vicinity  of  this  there  appear  in  embryos  of  24.4  mm.  five  small 
bud-like  downgrowths  of  the  epithelium  (Fig.  179,  SL),  which  later 
increase  considerably  in  number  as  well  as  in  size,  and  constitute  a 


THE    SALIVARY    GLANDS 


295 


group  of  glands  which  are  generally  spoken  of  as  the  sublingual 
gland. 

As  these  representatives  of  the  various  glands  increase  in 
length,  they  become  lobed  at  their  deeper  ends,  and  the  lobes 
later  give  rise  to  secondary  outgrowths  which  branch  repeatedly, 
the  terminal  branches  becoming  the  alveoli  of  the  glands.  A 
lumen  early  appears  in  the  duct  portions  of  the  structures,  the 
alveoli  remaining  solid  for  a  longer  time,  although  they  eventually 
also  become  hollow. 


j:ai 


Mart. 


Pig.  179. — Transverse  Section  of  the  Lower  Jaw  and  Tongue  of  an  Embryo 

of  about  20  mm. 
D,  Digastric  muscle;  GGl.,  genioglossus,  GH.\  geniohyoid;  I.Al,  inferior  alveolar 
nerve;  Man,  mandible;  MK.  Meckel's  cartilage;  My,  mylohyoid;  SL,  sublingual 
gland;  S.Mx,  submaxillary  duct;  T,  tongue. 

It  is  to  be  noted  that  each  parotid  and  submaxillary  consists  of  a 
single  primary  outgrowth,  and  is  therefore  a  single  structure  and  not  a 
union  of  a  number  of  originally  separate  parts.  The  sublingual  glands 
of  adult  anatomy  are  usually  described  as  opening  upon  the  floor  of 
the  mouth  by  a  number  of  separate  ducts.  This  arises  from  the 
fact  that  the  majority  of  the  glands  which  form  in  the  vicinity  of  the 
opening  of  Wharton's  duct  remain  quite  small,  only  one  of  them  on 
each  side  giving  rise  to  the  sublingual  gland  proper.  The  small  glands 
have  been  termed  the  alveolo-lingual  glands,  and  each  one  of  them  is 
equivalent  to  a  parotid  or  submaxillary  gland.  In  other  words,  there 
are  in  reality  not  three  pairs  of  salivary  glands,  but  from  fourteen  to 
sixteen  pairs,  there  being  usually  from  eleven  to  thirteen  alveolo-lingual 
glands  on  each  side. 


296  THE   PHARYNX 

The  Development  of  the  Pharynx. — The  pharynx  represents 
the  most  anterior  part  of  the  archenteron,  that  portion  in  which 
the  branchial  arches  develop,  and  in  the  embryo  it  is  relatively 
much  longer  than  in  the  adult,  the  diminution  being  brought  abou  t 
by  the  folding  in  of  the  posterior  arches  and  the  formation  of  the 
sinus  praecervicahs  already  described  (p.  100).  Between  the  vari- 
ous branchial  arches,  grooves  occur,  representing  the  endodermal 
portions  of  the  grooves  which  separate  the  arches.  During  devel- 
opment the  first  of  these  becomes  converted  into  the  tympanic 
cavity  of  the  ear  and  the  Eustachian  tube  (see  Chapter  XV) ;  the 
second  disappears  in  its  upper  part,  the  lower  persisting  as  the 
fossa  in  which  the  tonsil  is  situated;  while  the  lower  parts  of  the 
remaining  two  are  represented  by  the  sinus  piriformis  of  the  larynx 
(His),  and  also  leave  traces  of  their  existence  in  detached  portions 
of  their  epithelium  which  form  what  are  termed  the  branchial 
epithelial  bodies,  and  take  part  in  the  formation  of  the  thyreoid 
and  thymus  glands. 

In  the  floor  of  the  pharynx  behind  the  thickenings  which  pro- 
duce the  tongue  there  is  to  be  found  in  early  stages  a  pair  of  thick- 
enings passing  horizontally  backward  and  uniting  in  front  so  that 
they  resemble  an  inverted  U  (Fig.  180,  /).  These  ridges,  which 
form  what  is  termed  thefurcula  (His),  are  concerned  in  the  forma- 
tion of  parts  of  the  larynx  (see  p.  357).  In  the  part  of  the  roof  of 
the  pharynx  which  comes  to  lie  between  the  openings  of  the  Eusta- 
chian tubes,  a  collection  of  lymphatic  tissue  takes  place  beneath 
the  mucous  membrane,  forming  the  pharyngeal  tonsil,  and  imme- 
diately behind  this  there  is  formed  in  the  median  line  an  upwardly 
projecting  pouch,  the  pharyngeal  bursa,  first  certainly  noticeable 
in  embryos  6.5  mm.  in  length. 

This  bursa  has  very  generally  been  regarded  as  the  persistent  re- 
mains of  Rathke's  pouch  (p.  287),  especially  since  it  is  much  more 
pronounced  in  fetal  than  in  adult  life.  It  has  been  shown,  however, 
that  it  is  formed  quite  independently  of  and  posterior  to  the  true 
Rathke's  pouch  (Killian),  and  Huber's  observations  show  that  in  man 
it  represents  a  region  of  the  pharyngeal  epithelium  with  which  the  noto- 
chord  retains  connection  after  it  has  elsewhere  separated  from  the  endo- 
derm.     The  epithelium  becomes  thickened  at  the  point  of  contact  with 


THE  BRAJTCHIAL   EPITHELIAL  BODIES  297 

the  notochord  and  is  later  drawn  out  into  a  pouch  or  bursa,  which 
usually  disappears  after  birth,  but  may  result  in  the  formation  of  a  cyst 
in  the  roof  of  the  pharynx.  Structures  that  have  been  identified  witlT 
the  pharyngeal  bursa  in  the  embryos  of  other  mammals,  such  as  the  pig, 
are,  however,  formed  independently  of  any  contact  of  the  notochord 
with  the  pharyngeal  epithelium. 

The  tonsils  are  formed  from  the  epithelium  of  the  second  bran- 
chial groove.  At  about  the  fourth  month  solid  buds  begin  to 
grow  from  the  epithelium  into  the  subjacent  mesenchyme,  and 
depressions  appear  on  the  surface  of  this  region.  Later  the  buds 
become  hollow  by  a  cornification  of  their  central  cells,  and  open 
upon  the  floor  of  the  depressions  which  represent  the  crypts  of 
the  tonsil.  In  the  meantime  lymphocytes,  concerning  whose 
origin  there  is  a  difference  of  opinion,  collect  in  the  subjacent 
mesenchyme  and  eventually  aggregate  to  form  lymphatic  folli- 
cles in  close  relation  with  the  buds.  Whether  the  lymphocytes 
wander  out  from  the  blood  into  the  mesenchyme  or  are  derived 
directly  from  the  epithelium  or  the  mes- 
enchyme cells  is  the  question  at  issue. 

The  tonsil  may  grow  to  a  size  sufficient 
to  fill  up  completely  the  groove  in  which 
it  forms,  but  not  infrequently  a  marked 
depression,  the  fossa  supratonsillaris,  ex- 
ists above  it  and  represents  a  portion  of 
the  original  second  branchial  furrow. 

The  groove  of  Rosenmuller,  which  was 
at  one  time  thought  to  be  also  a  remnant 
of  the  second  furrow,  is  a  secondary  de-         fig.  i so.— The  Floor 
pression  which  appears  in  embryos  of  11.5   ZJZ  o^a'^rMM."'   '" 
cm.  behind  the  opening  of  the  Eustachian    f^  Furcuia;  /.  tubercuium 
tube,   in  about  the  region  of  the  third  impar— (His.) 

branchial  furrow. 

The  Development  of  the  Branchial  Epithelial  Bodies. — These  are 
structures  which  arise  either  as  thickenings  or  as  outpouchings  of 
the  epithelium  lining  the  lower  portions  of  the  inner  branchial 
furrows.  Five  pairs  of  these  structures  are  developed  and,  in 
addition,  there  is  a  single  unpaired  median  body.     This  last  makes 


298 


THE   BRANCHIAL   EPITHELIAL  BODIES 


its  appearance  in  embryos  of  about-  3  mm.,  and  gives  rise  to  the 
major  portion  of  the  thyreoid  body.  It  is  situated  immediately 
behind  the  anterior  portion  of  the  tongue,  at  the  apex  of  the  groove 
between  this  and  the  posterior  portion,  and  is  first  a  sUght  pouch- 
like depression.  As  it  deepens,  its  extremity  becomes  bilobed,  and 
after  the  embryo  has  reached  a  length  of  7  mm.  it  becomes  com- 
pletely separated  from  the  floor  of  the  pharynx.  The  point  of  its 
original  origin  is,  however,  permanently  marked  by  a  circular 
depression,  the  foramen  ccecum  (Fig.  177, /c).  Later  the  bilobed 
body  migrates  down  the  neck  and  becomes  a  solid  transversely 
elongated  mass  (Fig.  181,  th),  into  the  substance  of  which  tra- 


FiG.   181. — Reconstructions  of  the  Branchial  Epithelial  Bodies  of  Embryos. 

OF    (A)     14    MM.    AND    (B)     26    MM. 

ao.    Aorta;    Ith,  lateral  thyreoid;  ph,  pharynx;  pth^    and    pth^,   parathyreoids; //?, 
thyreoid;  thy,  thymus;  vc,  vena  cava  superior. — (Tourneux  and  Verdun.) 

beculae  of  connective  tissue  extend,  dividing  it  into  a  network  of 

anastomosing  cords  which  later  divide  transversely  to  form  follicles. 

When  the  embryo  has  reached  a  length  of  2.6  cm.,   a  cylindrical 

outgrowth  arises  from  the  anterior  surface  of  the  mass,  usually 

a  Httle  to  the  left  of  the  median  line,  and  extends  up  the  neck  a 

varying  distance,  forming,  when  it  persists  until  adult  life,  the 

so-called  pyramid  of  the  thyreoid  body. 

This  account  of  the  pyramid  follows  the  statements  made  by  recent 
workers  on  the  question  (Tourneux  and  Verdun);  His  has  claimed 
that  it  is  the  remains  of  the  stalk  connecting  the  thyreoid  with  the  floor 
of  the  pharynx,  and  which  he  terms  the  thyreo-glossal  duct. 

Two  other  pairs  of  bodies  enter  into  the  intimate  relations  with 


THE  BRANCHIAL   EPITHELIAL  BODIES 


299 


pthmlV 


ihmlV 


pthm  III 


the  thyreoid,  forming  what  have  been  termed  the  parathyreoid 
bodies  (Fig.  181,  pth^  and  pth"^).  One  of  these  pairs  arises  as^ 
thickening  of  the  dorsal  portion  of  the  fourth  branchial  groove  and 
the  other  comes  from  the  corresponding  portion  of  the  third  groove. 
The  members  of  the  former  pair,  after  separating  from  their  points 
of  origin,  come  to  lie  on  the 
dorsal  surface  of  the  lateral 
portions  of  the  thyreoid  body 
(Fig.  182,  pthm  IV)  in  close 
proximity  to  the  lateral  thy- 
reoids, while  those  of  the  other 
pair,  passing  further  back- 
ward, come  to  rest  behind  the 
lower  border  of  the  thyreoid 
(Fig.  182,  pthm  III).  The 
cells  of  these  bodies  do  not 
become  divided  into  cords  by 
the  ingrowth  of  connective 
tissue  to  the  same  extent  as 
those  of  the  thyreoids,  nor  do 
they  become  separated  into 
follicles,  so  that  the  bodies  are 
readily  distinguishable  by  their 
structure  from  the  thyreoid. 

From  the  ventral  portion 
of  the  third  branchial  groove 
a  pair  of  evaginations  de- 
velop, similar  to  those  which 
produce  the  lateral  thyreoids. 
These  elongate  greatly,  and 
growing  downward  ventrally 
to  the  thyreoid  and  separating  from  their  points  of  origin, 
come  to  lie  below  the  thyreoids,  forming  the  thymus  gland  (Fig. 
181,  thy).  As  development  proceeds  they  pass  further  backward 
and  come  eventually  to  rest  upon  the  anterior  surface  of  the 
pericardium.     The  cavity  which  they  at  first  contain  is  early 


thmlli 


Fig.    182. — Thyreoid,    Thymus   and  Epi- 
thelial Bodies  of  a  New-born  Child. 
pthm  III  and  pthm  IV,  Parathyreoids ; 
sd,    thyreoid;    thm    III,  thymus;    thm  IV, 
lateral  thyreoid. — (Groschuff.) 


300 


THE   BRANCHIAL   EPITHELIAL  BODIES 


obliterated  and  the  glands  assume  a  lobed  appearance  and  become 
traversed  by  trabeculae  of  connective  tissue.  Lymphocytes,  de- 
rived, according  to  some  recent  observations,  directly  from  the 
epithelium  of  the  glands,  make  their  appearance  and  gradually 
increase  in  number  until  the  original  epithelial  cells  are  represented 
only  by  a  number  of  peculiar  spherical  structures,  consisting  of 
cells  arranged  in  concentric  layers  and  known  as  HassaWs  cor- 
puscles. 


Pig.  183. — Diagram  showing  the  Origin  of  the  Various  Branchial  Epithelial 

Bodies. 

Ith,  Lateral  thyreoids;  pp,  ultimobranchial  bodies;  pht^  and  phi^,  pjarathyreoids; //j, 

median  thyreoid;  thy,  thymus;  I  to  IV,  branchial  grooves. — (Kohn.) 

The  glands  increase  in  size  until  about  the  fifteenth  year,  after 
which  they  gradually  undergo  degeneration  into  a  mass  of  fibrous 
and  adipose  tissue. 

A  pair  of  evaginations  very  similar  to  those  that  give  rise  to  the 
thymus  are  also  formed  from  the  ventral  portion  of  the  fourth 
branchial  groove  (Figs.  181,  ^  and  183,  Ith).  As  a  rule  they  com- 
pletely disappear  in  later  stages  of  development,  but  occasionally 
they  undergo  differentiation  into  small  masses  of  thymus-like 
tissue,  which  remain  associated  with  the  parathyreoids  from  the 


THE    CESOPHAGUS  3OI 

same  arch  (Fig.  182,  thm  IV).  They  have  been  termed  lateral 
thyreoids,  but  the  term  is  a  misnomer,  since  they  take  no  essential 
part  in  the  formation  of  the  thyreoid  body. 

Finally,  a  pair  of  outgrowths  arise  from  the  floor  of  the  pharynx 
just  behind  the  fifth  branchial  arch,  in  the  region  where  the  fifth 
groove,  if  developed,  would  occur.  These  ultimo-branchial  bodies, 
as  they  have  been  called,  usually  undergo  degeneration  at  an  early 
stage  and  disappear  completely,  though  occasionally  they  persist 
as  cystic  structures  embedded  in  the  substance  of  the  thyreoid. 

The  relation  of  these  various  structures  to  the  branchial  grooves  is 
shown  by  the  annexed  diagram  (Fig.  183),  and  from  it,  it  will  be  seen 
that  the  bodies  derived  from  the  third  and  fourth  grooves  are  serially 
equivalent.  Comparative  embryology  makes  this  fact  still  more 
evident,  since,  in  the  lower  vertebrates,  each  branchial  groove  contrib- 
utes to  the  formation  of  the  thymus  gland.  The  terminology  used 
above  for  the  various  bodies  is  that  generally  applied  to  the  mammalian 
organs,  but  it  would  be  better,  for  the  sake  of  comparison  with  other 
vertebrates,  to  adopt  the  nomenclature  proposed  by  Groschuff,  who 
terms  each  lateral  thyreoid  a  thymus  IV,  while  each  thymus  lobe  is  a 
thymus  III.  Similarly  the  para  thyreoids  are  termed  para  thymus 
and  III  and  IV,  the  term  thyreoid  being  limited  to  the  median  thryeoid. 

The  Musculature  of  the  Pharynx. — The  pharynx  differs  frohi 
other  portions  of  the  archenteron  in  the  fact  that  its  walls  are  fur- 
nished with  voluntary  muscles,  the  principal  of  which  are  the  con- 
strictors and  the  stylo-pharyngeus.  This  pecuHarity  arises  from 
the  relations  of  the  pharynx  to  the  branchial  arches.  It  has  been 
seen  that  in  the  higher  mammalia  the  dorsal  ends  of  the  third, 
fourth,  and  fifth  branchial  cartilages  disappear;  the  muscles  origi- 
nally associated  with  these  structures  persist,  however,  and  give  rise 
to  the  muscles  of  the  pharynx,  which  consequently  are  innervated 
by  the  ninth  and  tenth  nerves. 

The  Development  of  the  (Esophagus.— ^From  the  ventral  side 
of  the  lower  portion  of  the  pharynx  an  evagination  develops  at 
an  early  stage,  which  is  destined  to  give  rise  to  the  organs  of  respi- 
ration; the  development  of  this  may,  however,  be  conveniently 
postponed  to  a  later  chapter  (Chapter  XII). 

The  oesophagus  is  at  first  a  very  short  portion  of  the  archen- 
teron (Fig.  184,  A),  but  as  the  heart  and  diaphragm  recede  into 


302 


THE    STOMACH 


the  thorax,  it  elongates  (Fig.  184,  B)  until  it  eventually  forms  a 
considerable  portion  of  the  digestive  tract.  Its  endodermal  lining 
like  that  of  the  rest  of  the  digestive  tract  except  the  pharynx,  is 
surrounded  by  splanchnic  mesoderm  whose  cells  become  converted 
into  non-striated  muscular  tissue,  which,  by  the  fourth  month,  has 
separated  into  an  inner  circular  and  an  outer  longitudinal  layer. 
The  Development  of  the  Stomach  and  Intestines. — By  the 
time  the  embryo  has  reached  a  length  of  about  5  mm.  its  constric- 


FiG.  184. — Reconstruction  of  the  Digestive  Tract  of  Embryos  of  (A)  4.2  mm. 

AND  (B)   5  MM. 

all,  Allantois;  cl,  cloaca;  I,  lung;  It,  liver;  Rp,  Rathke's  pouch;  S,  stomach;  /,  tongue; 

th,  thyreoid  body;  Wd,  Wolffian  duct;  y,  yolk-stalk. — (His.) 

tion  from  the  yolk-sac  has  proceeded  so  far  that  a  portion  of  the 
digestive  tract  anterior  to  the  yolk-sac  can  be  recognized  as  the 
stomach  and  a  portion  posterior  as  the  intestine.  As  first  the 
stomach  is  a  simple,  spindle-shaped  enlargement  (Fig.  184)  and 
the  intestine  a  tube  without  any  coils  or  bends,  but  since  in  later 


THE   INTESTINE  3O3 

stages  the  digestive  tract  grows  much  more  rapidly  in  length  than 
the  abdominal  cavity,  a  coiling  of  the  intestine  becomes  necessary. 

The  elongation  of  the  stomach  early  produces  changes  in  its 
position,  its  lower  end  bending  over  toward  the  right,  while  its  up- 
per end,  owing  to  the  development  of  the  liver,  is  forced  somewhat 
toward  the  left.  At  the  same  time  the  entire  organ  undergoes  a 
rotation  about  its  longitudinal  axis  through  nearly  ninety  degrees, 
so  that,  as  the  result  of  the  combination  of  these  two  changes,  what 
was  originally  its  ventral  border  becomes  its  lesser  curvature  and 
what  was  originally  its  left  surface  becomes  its  ventral  surface. 

Hence  it  is  that  the  left  vagus  nerve  passes  over  the  ventral  and 
the  right  over  the  dorsal  surface  of  the  stomach  in  the  adult. 


Fig.  185. — The  Gastric  Epithelium  from  an  Embryo  of  10  mm.     From  a  Re- 
construction. 
D.ch,  Common  bile  duct;  D.p.d,  duct  of  dorsal  pancreas;  I.ang,  angular  notch  be- 
tween cardiac  and  pyloric  portions;  Oe,  oesophagus. — (F.  T.  Lewis.) 

In  the  meantime  the  elongation  of  the  oesophagus  has  carried 
the  stomach  further  away  from  the  lower  end  of  the  pharynx, *and 
from  being  spindle-shaped  it  has  become  more  pyriform,  the  fundus 
having  appeared  as  an  outpouching  of  the  wall.  At  first  the  py- 
loric portion  is  almost  half  the  length  of  the  entire  organ  (Fig.  185), 
but  later  it  becomes  relatively  shorter  and  the  region  of  the  pylorus 
becomes  marked  as  a  slight  dilation.  It  is  worthy  of  note  that  two 
folds  of  mucous  membrane  are  early  to  be  seen  extending  from  the 
opening  of  the  oesophagus  along  the  lesser  curvature  to  the  com- 
mencement of  the  pyloric  portion.     They  form  the  walls  of  a 


304 


THE    INTESTINE 


groove  (Fig.  185),  which  may  be  recognized  in  the  adult  stomach 
and  to  which  the  term  gastric  canal  has  been  apphed  (F.  T.  Lewis) . 
The  growth  of  the  intestine  results  in  its  being  thrown  into  a 
loop  opposite  the  point  where  the  yolk-stalk  is  still  connected  with 
it,  the  loop  projecting  ventrally  into  the  portioA  of  the  coelomic 
cavity  which  is  contained  within  the  umbilical  cord,  and  being 
placed  so  that  its  upper  Hmb  lies  to  the  right  of  the  lower  one.  Up- 
on the  latter  a  slight  pouch-like  lateral  outgrowth  appears  which 


Pig.  186. — Reconstruction  of  Embryo  of  20  mm. 
C,  Caecum;  K,  kidney;  L,  liver;  S,  stomach;  SC,  suprarenal  bodies;  W,  mesonephros. 

(Mall.) 

is  the  beginning  of  the  ccBcum  and  marks  the  line  of  union  of  the 
future  small  and  large  intestine.  The  small  intestine,  continuing 
to  lengthen  more  rapidly  than  the  large,  assumes  a  sinuous  course 
(Fig.  186),  in  which  it  is  possible  to  recognize  six  primary  coils 
which  continue  to  be  recognizable  until  advanced  stages  of  develop- 
ment and  even  in  the  adult  (Mall).  The  first  of  these  is  at  first 
indistinguishable  from  the  pyloric  portion  of  the  stomach  and  can 


THE    INTESTINE  305 

be  recognized  as  the  duodenum  only  by  the  fact  that  it  has  con- 
nected with  it  the  ducts  of  the  Hver  and  pancreas;  as  development 
proceeds,  however,  its  caliber  diminishes  and  it  assumes  the  ap- 
pearance of  a  portion  of  the  intestine. 

The  remaining  coils  elongate  rapidly  and  are  thrown  into 
numerous  secondary  coils,  all  of  which  are  still  contained  within 
the  coelom  of  the  umbilical  cord  (Fig.  187) .  When  the  embryo  has 
reached  a  length  of  about  40  mm.  the  coils  rather  suddenly  return 
to  the  abdominal  cavity,  and  now  the  caecum  is  thrown  over  to- 
ward the  right,  so  that  it  comes  to  lie  immediately  beneath  the 


Fig.  187. — Reconstruction  of  the  Intestine  of  an  Embryo  of  19  mm.     The 
Figures  on  the  Intestine  Indicate  the  Primary  Coils. — (Mall.) 

liver  on  the  right  side  of  the  abdominal  cavity,  a  position  which  it 
retains  until  about  the  fourth  month  after  birth  (Treves).  The 
portion  of  the  large  intestine  which  formerly  projected  into  the 
umbihcal  coelom  now  lies  transversely  across  the  upper  part  of  the 
abdomen,  crossing  in  front  of  the  duodenum  and  having  the  re- 
maining portion  of  the  small  intestine  below  it.  The  elongation 
continuing,  the  secondary  coils  of  the  small  intestine  become  more 
numerous  and  the  lower  portion  of  the  large  intestine  is  thrown 
into  a  loop  which  extends  transversely  across  the  lower  part  of  the 
abdominal  cavity  and  represents  the  sigmoid  flexure  of  the  colon . 

At  the  time  of  birth  this  portion  of  the  large  intestine  is  relatively 
20 


3o6 


THE   INTESTINE 


much  longer  than  in  the  adult,  amounting  to  nearly  half  the  entire 
length  of  the  colon  (Treves),  but  after  the  fourth  month  after 
birth  a  readjustment  of  the  relative  lengths  of  the  parts  of  the 
colon  occurs,  the  sigmoid  flexure  becoming  shorter  and  the  rest  of 
the  colon  proportionally  longer,  whereby  the  caecum  is   pushed 


Fig.   1 88. — Representation  of  the  Coilings  of  the  Intestine  in  the  Adult 
Condition.     The  Numbers  indicate  the  Primary  Coils, — (Mall.) 

downward  until  it  lies  in  the  right  iliac  fossa,  the  ascending  colon 
being  thus  established.. 

When  this  condition  has  been  reached,  the  duodenum,  after 
passing  downward  for  a  short  distance  so  as  to  pass  dorsally  to  the 
transverse  colon,  bends  toward  the  left  and  the  secondary  coils 
derived  from  the  second  and  third  primary  coils  come  to  occupy 


THE   INTESTINE  307 

the  left  upper  portion  of  the  abdominal  cavity.  Those  from  the 
fourth  primary  coil  pass  across  the  middle  line  and  occupy  the- 
right  upper  part  of  the  abdomen,  those  from  the  fifth  cross  back 
again  to  the  left  lumbar  and  iliac  regions,  and  those  of  the  sixth 
take  possession  of  the  false  pelvis  and  the  right  iliac  region  (Fig. 
188). 

Slight  variations  from  this  arrangement  are  not  infrequent,  but  it 
occurs  with  sufficient  frequency  to  be  regarded  as  the  normal.  A 
failure  in  the  readjustment  of  the  relative  lengths  of  the  different  parts 
of  the  colon  may  also  occasionally  occur,  in  which  case  the  caecum  will 
retain  its  embryonic  position  beneath  the  liver. 

The  yolk-stalk  is  continuous  with  the  intestine  at  the  extremity 
of  the  loop  which  extends  out  into  the  umbilical  coelom,  and  when 
the  primary  coils  become  apparent  its  point 
of  attachment  lies  in  the  region  of  the  sixth 
coil.  As  a  rule,  the  caliber  of  the  stalk  does 
not  increase  proportionally  with  that  of  the 
intestine,  and  eventually  its  embryonic  por- 
tion disappears  completely.  Occasionally, 
however,  this  portion  of  it  does  partake  of 
the  increase  in  size  which  occurs  in  the  in- 
testine, and  forms  a  blind  pouch  of  vary- 
ing length,  known  as  MeckeVs  diverticulum  Fig.    189.— c^cum    of 

/  ^\  Embryo  of  10.2  cm, 

(see  p.  116).  „  .       .  „ 

^         ^  '  c.  Colon;  t,  ileum. 

The  ccBcum  has  been  seen  to  arise  as  a 
lateral  outgrowth  at  a  time  when  the  intestine  is  first  drawn  out 
into  the  umbilicus.  During  subsequent  development  it  continues 
to  increase  in  size  until  it  forms  a  conical  pouch  arising  from  the 
colon  just  where  it  is  joined  by  the  small  intestine  (Fig.  189). 
The  enlargement  of  its  terminal  portion  does  not  keep  pace,  how- 
ever, with  that  of  the  portion  nearest  the  intestine,  but  it  be- 
comes gradually  more  and  more  marked  off  from  it  by  its  lesser 
caliber  and  gives  rise  to  the  vermiform  appendix.  At  birth  the 
original  conical  form  of  the  entire  outgrowth  is  still  quite  evi- 
dent, though  it  is  more  properly  described  as  funnel-shaped,  but 
later  the  proximal  part,  continuing  to  increase  in  diameter  at  the 


3o8 


THE    LIVER 


same  rate  as  the  colon,  becomes  sharply  separated  from  the  ap- 
pendix, forming  the  caecum  of  adult  anatomy. 

Up  to  the  time  when  the  embryo  has  reached  a  length  of  14 
mm.,  the  inner  surface  of  the  intestine  is  quite  smooth,  but  when  a 
length  of  19  mm.  has  been  reached,  the  mucous  membrane  of  the 
upper  portion  becomes  thrown  into  longitudinal  folds,  and  later 
these  make  their  appearance  throughout  its  entire  length  (Fig. 
190).  Later,  in  embryos  of  60  mm.,  these  folds  break  up  into 
numbers  of  conical  processes,  the  villi,  which  increase  in  number 


Fig.  190. — Reconstruction  of  a  Portion  of  the  Intestine  of  an  Embryo 

OF  28  MM.  showing  THE  LONGITUDINAL  POLDS  FROM  WHICH  THE  ViLLI  ARE  PORMED. 

— (Berry.) 


with  the  development  of  the  intestine,  the  new  villi  appearing  in 
the  intervals  between  those  already  present.  Villi  are  formed 
as  well  in  the  large  as  in  the  small  intestine,  but  in  the  former  they 
decrease  in  size  as  development  proceeds  and  practically  dis- 
appear toward  the  end  of  fetal  life. 

In  the  early  stages  the  endodermal  lining  of  the  digestive  tract 
assumes  a  considerable  thickness,  the  lumen  of  the  oesophagus  and 
upper  part  of  the  small  intestine  being  reduced  to  a  very  small  caliber. 
In  later  stages  a  rapid  increase  in  the  size  of  the  lumen  occurs,  appar- 
ently associated  with  the  formation  of  cavities  or  vacuoles  in  the 
endodermal  epithelium.  These  increase  in  size,  the  neighboring  cells 
arrange  themselves  in  an  epithelial  layer  around  their  walls  and  they 


THE   LIVER  309 

eventually  break  through  into  the  general  lumen.     They  are   some- 
times sufficiently  large  to  give  the  appearance  of  diverticula  of  the 
gut,  but  later  they  flatten  out,  their  cavities  becoming  portions  of  the" 
general  lumen. 

In  the  case  of  the  duodenum  the  thickening  of  the  endodermal 
lining  proceeds  to  such  an  extent  that  in  embryos  of  from  12.5  mm.  to 
14.5  mm.  the  lumen  is  completely  obliterated  immediately  below  the 
opening  of  the  hepatic  and  pancreatic  ducts.  This  condition  is  inter- 
esting in  connection  with  the  occasional  occurrence  in  new-born 
children  of  an  atresia  of  the  duodenum.  Under  normal  conditions, 
however,  the  lumen  is  restored  by  the  process  of  vacuolization  de- 
scribed above. 


Fig.  191. — Reconstruction  of  the  Liver  Outgrowths  of  Rabbit  Embryos  of 

(A)    5    MM.    AND    {B)    OF    8    MM, 

B,  Gall-bladder;  d,  duodenum;  DV,  ductus  venosus;  L,  liver;  p,  dorsal  pancreas;  pm, 
ventral  pancreas;  rL,  right  lobe  of  the  liver;  S,  stomach. — (Hammar.) 

The  Development  of  the  Liver.— The  liver  makes  its  appear- 
ance in  embryos  of  about  3  mm.  as  a  longitudinal  groove  upon  the 
ventral  surface  of  the  archenteron  just  below  the  stomach  and 
between  it  and  the  umbilicus.  The  endodermal  cells  lining  the 
anterior  portion  of  the  groove  early  undergo  a  rapid  proliferation, 
and  form  a  solid  mass  which  projects  ventrally  into  the  substance 
of  a  horizontal  shelf,  the  septum  transversum  (see  p.  321),  attached 
to  the  ventral  wall  of  the  body.  This  solid  mass  (Fig.  igi,  L) 
forms  the  beginning  of  the  liver  proper,  while  the  lower  portion  of 


310  THE    LIVER 

the  groove,  which  remains  hollow,  represents  the  future  gall- 
bladder (Fig.  191,  B).  Constrictions  appearing  between  the 
intestine  and  both  the  hepatic  and  cystic  portions  of  the  organ 
gradually  separate  these  from  the  intestine,  until  they  are  united 
to  it  only  by  a  stalk  which  represents  the  ductus  choledochus 
(Fig.  191). 

The  further  development  of  the  liver,  so  far  as  its  external 
form  is  concerned,  consists  in  the  rapid  enlargement  of  the  hepatic 
portion  until  it  occupies  the  greater  part  of  the  upper  half  of  the 
abdominal  cavity,  its  ventral  edge  extending  as  far  down  as  the 
umbilicus.  In  the  rabbit  its  substance  becomes  divided  into  four 
lobes  corresponding  to  the  four  veins,  umbilical  and  vitelline, 
which  traverse  it,  and  the  same  condition  occurs  in  the  human 
embryo,  although  the  lobes  are  not  so  clearly  indicated  upon  the 
surface  as  in  the  rabbit.  The  two  vitelline  lobes  are  in  close 
apposition  and  may  almost  be  regarded  as  one,  a  median  ventral 
lobe  which  embraces  the  ductus  venosus  (Fig.  igi,  B,  DV),  while 
the  umbilical  lobes  are  more  lateral  and  dorsal  and  represent  the 
right  {rL)  and  left  lobes  of  the  adult  liver.  The  remaining  definite 
lobes,  the  caudate  (Spigelian)  and  quadrate,  are  of  later  formation, 
standing  in  relation  to  the  vessels  which  cross  the  lower  surface  of 
the  liver. 

The  ductus  choledochus  is  at  first  wide  and  short,  and  near  its 
proximal  end  gives  rise  to  a  small  outgrowth  on  each  side,  one  of 
which  becomes  the  ventral  pancreas  (Fig.  igi,  B,  pm).  Later  the 
duct  elongates  and  becomes  more  slender,  and  the  gall-bladder  is 
constricted  off  from  it,  the  connecting  stalk  becoming  the  cystic 
duct.  The  hepatic  ducts  are  apparently  developed  from  the  Hver 
substance  and  are  relatively  late  in  appearing. 

Shortly  after  the  hepatic  portion  has  been  differentiated  its  sub- 
stance becomes  permeated  by  numerous  blood-vessels  (sinusoids) 
and  so  divided  into  anastomosing  trabeculae  (Fig.  192).'  These 
are  at  first  irregular  in  size  and  shape,  but  later  they  become  more 
slender  and  more  regularly  cylindrical^  forming  what  have  been 
termed  the  hepatic  cylinders.  In  the  center  of  each  cyHnder,  where 
the  cells  which  form  it  meet  together,  a  fine  canal  appears,  the 


THE    LIVER  311 

beginning  of  a  bile  capillary ^  the  cylinders  thus  becoming  converted 
into  tubes  with  fine  lumina.  This  occurs  at  about  the  fourth  week 
of  development  and  at  this  time  a  cross-section  of  a  cyHnder 
shows  it  to  be  composed  of  about  three  or  four  hepatic  cells  (Fig. 
193,  A),  among  which  are  to  be  seen  groups  of  smaller  cells  (e) 
which  are  erythrocytes,  the  liver  having  assumed  by  this  time  its 
haematopoietic  function  (see  p.  226).  This  condition  of  affairs 
persists  until  birth,  but  later  the  cylinders  undergo  an  elongation, 
the  cells  of  which  they  are  composed  slipping  over  one  another 


Fig.   192. — Transverse  Section  through  the  Liver  of  an  Embryo  of  Pour 

Months. 
in,  Intestine;  /,  liver;  W,  Wolffian  body. — {Toldt  and  Zuckerkandl.) 

apparently,  so  that  the  cylinders  become  thinner  as  well  as  longer 
and  show  for  the  most  part  only  two  cells  in  a  transverse  section 
(Fig.  193,  B)\  and  in  still  later  periods  the  two  cells,  instead  of 
lying  opposite  one  another,  may  alternate,  so  that  the  cylinders 
become  even  more  slender. 

The  bile  capillaries  seem  to  make  their  appearance  first  in  cylin- 
ders which  lie  in  close  relation  to  branches  of  the  portal  vein  (Fig. 
194),  and  thence  extend  throughout  the  neighboring  cyHnders, 
anastomosing  with  capillaries  developing  in  relation  to  neighboring 
portal  branches.     As  the  extension  so  proceeds  the  older  capillaries 


312 


THE    LIVER 


continue  to  enlarge  and  later  become  transformed  into  bile-duds 
(Fig.  194,  C),  the  cells  of  the  cylinders  in  which  these  capillaries 
were  situated  becoming  converted  into  the  epithelial  Hning  of  the 
ducts. 

The  lobules,  which  form  so  characteristic  a  feature  of  the  adult 
liver,  are  late  in  appearing,  not  being  fully  developed  until  some 
time  after  birth.  They  depend  upon  the  relative  arrangement  of 
the  branches  of  the  portal  and  hepatic  veins ;  these  at  first  occupy 
distinct  territories  of  the  liver  substance,  being  separated  from  one 
another  by  practically  the  entire  thickness  of  the  liver,  although  of 


Fig,  193. — Transverse  Sections  of  Portions  of  the  Liver  of  (A)  a  Fetus  of 

Six  Months  and  (B)  a  Child  of  Four  Years. 
be,  Bile  capillary;  e,  erythrocyte;  he,  hepatic  cylinder. — (Toldt  and  Zuckerkandl.) 


course  connected  by  the  sinusoidal  capillaries  which  He  between  the 
hepatic  cyHnders.  During  development  the  two  sets  of  branches 
extend  more  deeply  into  the  liver  substance,  each  invading  the 
territory  of  the  other,  but  they  can  readily  be  distinguished  from 
one  another  by  the  fact  that  the  portal  branches  are  enclosed 
within  a  sheath  of  connective  tissue  (Glisson's  capsule)  which  is 
lacking  to  the  hepatic  vessels.  At  about  the  time  of  birth  the 
branches  of  the  hepatic  veins  give  off  at  intervals  bunches  of 
terminal  vessels,  around  which  branches  of  the  portal,  vein  arrange 


THE   PANCREAS 


313 


themselves,  the  liver  tissue  becoming  divided  up  into  a  number  of 
areas  which  may  be  termed  hepatic  islands,  each  of  which  is  sur- 
rounded by  a  number  of  portal  branches  and  contains  numerous 
dichotomously  branching  hepatic  terminals.  Later  the  portal 
branches  sink  into  the  substance  of  the  islands,  which  thus  become 
lobed,  and  finally  the  sinking  in  extends  so  far  that  the  original 
island  becomes  separated  into  a  number  of  smaller  areas  or  lobules, 
each  containing,  as  a  rule,  a  single  hepatic  terminal  (the  intra- 
lobular vein)  and  being  surrounded  by  a  number  of  portal  terminals 
{interlobular  veins),  the  two  systems  being  united  by  the  capillaries 
which  separate  the  cylinders  contained  within  the  area.     The 


Fig.   194. — Injected  Bile  Capillaries  of  Pig  Embryos  of  (A)  8  cm.,  (B)  16  cm., 
AND  (C)  OF  Adult  Pig. — (Hendrickson.) 

lobules  are  at  first  very  small,  but  later  they  increase  in  size  by  the 
extension  of  the  hepatic  cylinders. 

Frequently  in  the  human  liver  lobules  are  to  be  found  containing 
two  intralobular  veins,  a  condition  with  results  from  an  imperfect 
subdivision  of  a  lobe  of  the  original  hepatic  island. 

The  liver  early  assumes  a  relatively  large  size ,  its  weight  at  one 
time  being  equal  to  that  of  the  rest  of  the  body,  and  though  in 
later  embryonic  stages  its  relative  size  diminishes,  yet  at  birth  it  is 
still  a  voluminous  organ,  occupying  the  greater  portion  of  the 
upper  half  of  the  abdominal  cavity  and  extending  far  over  into  the 
left  h3rpochondrium.  Just  after  birth  there  is,  however,  a  cessa- 
tion of  growth,  and  the  subsequent  increase  proceeds  at  a  much 
slower  rate  than  that  of  the  rest  of  the  body,  so  that  its  relative 


314 


THE  PANCREAS 


size  becomes  still  more  diminished  (see  Chap.  XVII).  The  cessa- 
tion of  growth  affects  principally  the  left  lobe  and  is  accompanied 
by  an  actual  degeneration  of  portions  of  the  liver  tissue,  the  cells 
disappearing  completely,  while  the  ducts  and  blood-vessels  origi- 
nally present  persist,  the  former  constituting  the  vasa  aherrantia 
of  adult  anatomy.  These  are  usually  especially  noticeable  at  the 
left  edge  of  the  liver,  between  the  folds  of  the  left  lateral  ligament, 
but  they  may  also  be  found  along  the  line  of  the  vena  cava,  around 
the  gall-bladder,  and  in  the  region  of  the  left  longitudinal  fissure. 
The  Development  of  the  Pancreas. — The  pancreas  arises  a 
little  later  than  the  liver,  as  two  or  three  separate  outgrowths,  one 


». 


Pig.  195. — Reconstruction  of  the  Stomach,  Duodenum  and  Pancreas  of  a 
Human  Embryo  of  17.8  mm.  (Thyng). 
A.Du,  Antrum  duodenale;  C,  stomach;  D.choL,  bile  duct;  D.cyst.,  cystic  duct; 
D.hep.,  hepatic  duct;  D.panc.d.,  dorsal  pancreatic  duct;  D.panc.v.,  ventral  pancreatic 
duct;  F,  fundus  of  stomach;  CE,  oesophagus;  Panc.d.,  dorsal  pancreas;  Panc.v.,  ven- 
tral pancreas;  P.py.,  pyloric  portion  of  stomach. 

from  the  dorsal  surface  of  the  duodenum  (Fig.  195,  Panc.d)  usu- 
ally a  little  above  the  liver  outgrowth,  and  one  or  two  from  the 
lower  part  of  the  common  bile-duct .  Of  the  latter  outgrowths ,  that 
upon  the  left  side  may  be  wanting  and,  if  formed,  early  disappears, 
while  that  of  the  right  side  {Panc.v.)  continues  its  development 


LITERATURE  315 

to  form  what  has  been  termed  the  ventral  pancreas.  Both 
this  and  the  dorsal  pancreas  continue  to  elongate,  the  latter  lying 
to  the  left  of  the  portal  vein,  while  the  former,  at  first  situated  to 
the  right  of  the  vein,  later  grows  across  its  ventral  surface  so  as  to 
come  into  contact  with  the  dorsal  gland,  with  which  it  fuses  so 
intimately  that  no  separation  line  can  be  distinguished.  The 
body  and  tail  of  the  adult  pancreas  represent  the  original  dorsal 
outgrowth,  while  the  right  ventral  pancreas  becomes  the  head. 

Both  the  dorsal  and  ventral  outgrowths  early  become  lobed, 
and  the  lobes  becoming  secondarily  lobed  and  this  lobation  re- 
peating itself  several  times,  the  compound  tubular  structure  of 
the  adult  gland  is  acquired,  the  very  numerous  terminal  lobules 
becoming  the  secreting  acini,  while  the  remaining  portions  become 
the  ducts.  Of  the  principal  ducts,  there  are  at  first  two;  that  of 
the  dorsal  pancreas,  the  duct  of  Santorini,  opens  into  the  duode- 
num on  its  dorsal  surface,  while  that  of  the  ventral  outgrowth,  the 
duct  of  Wirsung,  opens  into  the  ductus  choledochus.  When  the 
fusion  of  the  two  portions  of  the  gland  occurs,  an  anastomosis  of 
branches  of  the  two  ducts  develops  and  the  proximal  portion  of  the 
duct  of  Santorini  may  degenerate,  so  that  the  secretion  of  the 
entire  gland  empties  into  the  common  bile-duct  through  the  duct 
of  Wirsung. 

In  the  connective  tissue  which  separates  the  lobules  of  the 
gland,  groups  of  cells  occur,  which^have  no  connection  with  the 
ducts  of  the  gland,  and  form  what  are  termed  the  areas  of  Langer- 
hans.  They  arise  by  a  differentiation  of  the  cells  which  form  the 
original  pancreatic  outgrowths,  and  have  been  distinguished  in  the 
dorsal  pancreas  of  the  guinea-pig  while  it  is  still  a  solid  outgrowth. 
They  gradually  separate  from  the  remaining  cells  of  the  out- 
growth and  come  to  lie  in  the  mesenchyme  of  the  gland  in  groups 
into  which,  finally,  blood-vessels  penetrate. 

LITERATURE 

H.  Ahrens:  "Die  Entwicklung  der  menschliche  Zahne,"  Anat.  Hefte,  xxviii,  1913. 
E.  T.  Bell:  "The  Development  of  the  Thymus,"  Amer.  Journ.  of  Anat.,  v,  1906. 
J,  M.  Berry:  "On^the  Development  of  the  Villi  of  the  Human  Intestine,"  AncU.y^ 
Anzeiger,  xvi,  1900. 


3l6  LITERATURE 

Gertrud  Bien:  "Zur  Entwicklungsgeschichte  des  menschlichen  Dickdarms,"  Anat. 

Hefte,  XLix,  1913. 
L.  Bolk:  "Die  Entwicklungsgeschichte  der  menschlichen  Lippen,"  Anat.  Hefte, 

XLiv,  1908. 
L,  Bolk:  "Ueber  die  Gaumenentwicklung  und  die  Bedeutung  der  oberen  Zahn- 

leiste  beim  Menschen,"  Zeit.filr  Morphol.  und  AnthropoL,  xiv,  191 1. 
J.  Bracket:  "Recherches  sur  le  d^veloppement  du  pancreas  et  du  foie,"  Journ.  de 

VAnat.  et  de  la  Physiol.,  xxxn,  1896. 
O.  C.  Bradley:  "A  Contribution  to  the  Morphology  and  Development  of  the  Mam- 
malian Liver,"  Journ.  Anat.  and  Physiol.,  XLiir,  1908. 
H.  M.  DE  Burlet:  "Die  ausseren  Formverhaltnisse  der  Leber  beim  menschlichen 

Embryo,"  Morphol.  Jahrb.,  xlii,  1910. 
■  R.  V.  Chamberlin:  "On  the  Mode  of  Disappearance  of  the  Villi  from  the  Colon  of 

Mammals,"  Anat.  Record,  iii,  1909. 
J.  H.  Chievitz:  "Beitrage  zur  Entwicklungsgeschichte  der  Speicheldriisen,"  Archiv 

fur  Anat.  und  Physiol.,  Anat.  Ahth.,  1885. 
H.  Fox:  "The  Pharyngeal  Pouches  and  Their  Derivatives  in  the  Mammalia,"  Amer. 

Journ.  Anat.,  viii,  1908. 
K.  Groschuff:  "Ueber  das  Vorkommen  eines  Thymussegementes  der  vierten  Kie- 

mentasche  beim  Menschen,"  Anat.  Anzeiger,  xvii,  1900. 
O.  Grosser:  "Zur  Kenntnis  des  ultimobranchiaJen  Korpers  beim  Menschen,"  Anat. 

Anzeiger,  xxxvir,  1910. 
L.  Grijnwald:  "Ein  Beitrag  zur  Entstehung  und  Bedeutung  der  Gaumenmandeln," 

Anat.  Anzeiger,  xxxvii,  19 10. 
J.  A.  Hammar:  "Einige  Plattenmodelle  zur  Beleuchtung  der  friiheren  embryonal 

Leberentwicklung,"  Arch.f.  Anat.  undPhys.,  Anat.  Ahth.,  1893. 
J.  A.  Hammar:  "Notiz  iiber  die  Entwicklung  der  Zunge  und  der  Mundspeichel- 

drusen  beim  Menschen,"  Anat.  Anzeiger,  xix,  1901. 
J.  A.  Hammar:  "Studien  tiber  die  Entwicklung  des  Vorderdarms  und  einiger  angren- 

zender  Organe,"  Arch.  f.  mikrosk.  Anat.,  lix  and  lx,  1902. 
K.   Helly:  "Zur   Entwickelungsgeschichte  der  Pancreasanlagen  und  Duodenal- 

papillen  des  Menschen,"  Archiv  fUr  mikrosk.  Anat.,  lvi,  1900. 
K.  Helly:  "Studien  iiber  Langerhanssche  Insein,"  Arch.  fUr  mikrosk.  Anat.,  lxvii, 

1907. 
W.  F.  Hendrickson:  "The  Development  of  the  Bile-capillaries  as  revealed  by 

Golgi's  Method,"  Johns  Hopkins  Hospital  Bulletin,  1898. 
W.  His:  "Anatomie  menschlicher  Embryonen,"  Leipzig,  1882-1886. 

F.  Hochstetter:  "Ueber  die  Bildung  der  primitiven  Choanen  beim  Menschen," 

Anat.  Anzeiger,  vri,  1892. 
H.  Holmdahl:  "Zur  Entwicklungsgegchichte  des  menschlichen  Rectums,"  Anat. 
Hefte,  LI,  1914. 

G.  C.  Huber:  "On  the  Relations  of  the  Chorda  Dorsalis  to  the  Pharyngeal  Bursa  or 

Median  Pharyngeal  Recess,"  Anat.  Record,  vi,  1912. 

N.  W.  Ingalls:  "A  Contribution  to  the  Embryology  of  the  Liver  and  Vascular 
System  in  Man,"  Anat.  Record,  n,  1908. 

C.  M.  Jackson:  "On  the  Development  and  Topography  of  the  Thoracic  and  Abdom- 
inal Viscera,"  Anat.  Record,  in,  1909. 


LITERATURE  317 

¥.  P.  Johnson:  "The  Development  of  the  Mucous  Membrane  of  the  (Esophagus, 
Stomach  and  Small  Intestine  in  the  Human  Embryo,"  Amer.  Journ.  Anat.,  x, 
1910.  ^-: — 

F.  P.  Johnson:  "The  Development  of  the  Mucous  Membrane  of  the  Large  Intestine 
and  Vermiform  Process  in  the  Human  Embryo,"  Amer.  Journ.  Anat.,  xiv,  1913. 
-F.  P.  Johnson:  "The  Development  of  the  Rectum  in  the  Human  Embryo,"  Amer. 
Journ.  Anat.,  xvi,  1914. 

E.  Kallius:  "Beitrage  zur  Entwicklung  der  Zunge,  3te  Th.  Saugetiere.  I.  Sus 

scrofa,"  Anat.  Hefte,  xli,  1910. 

F.  Keibel:  "Zur  Entwickelungsgeschichte  des  menschlichen  Urogenital-apparatus," 

Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1896. 

G.  Killian:  "Ueber  die  Bursa  und  Tonsilla  phaiyngea.,^'  Morphol.  J ahrbuch,xiv, 

1888. 
B.  F.  Kingsbury:  "On  the  so-called  ultimobranchial  body  of  the  mammalian 

embryo:  man,  Anat.  Anzeiger,XL\ii,  1914. 
B.  F.  Kingsbury:  "The  development  of  the  human  pharynx,  i,  The  pharyngeal 

derivatives,"  Amer.  Journ.  Anat.,  xviii,  1915. 
A.  Kohn:  "Die  Epithelkorperchen,"  Ergebnisse  der  Anat.  und  Entwicklungsgesch., 

IX,  1899. 

E.  Kreuter:  "Die  angeborenen  Verschliessungen  und  Verengerungen  des  Darm- 

kanals  in  Lichte  der  Entwicklungsgeschichte,"  Deutsches  Zeitschr.  f.  Chir.j 
Lxxrx,  1905. 
H.  Kuster:  "Zur  Entwicklungsgeschichte  der  Langerhans'schen  Inseln  im  Pancreas 
beim  menschlichen  Embryo,"  Arch,  fiir  mikrosk.  Anat.,  lxiv,  1904. 

F.  T.  Lewis:  "The  Form  of  the  Stomach  in  Human  Embryos,  etc."     Amer.  Journ. 

Anat.,  xiii,  1912. 
-F.  T.  Lewis  and  F.  W.  Thyng:  "The  Regular  Occurrence  of  Intestinal  Diverticula 

in  Embryos  of  the  Pig,  Rabbit  and  Man,"  Amer.  Journ.  Anat.,  vii,  1908. 
F.  P.  Mall:  "Ueber  die  Entwickelung  des  menschlichen  Darmes  und  seiner  Lage 

beim  Erwachsenen,"  Archiv  Jiir  Anat.  und  Physiol.,  Anat.  Abth.,  Supplement, 

1897. 
F.  P.  Mall:  "A  Study  of  the  Structural  Unit  of  the  Liver,"  Amer.  Journ.  of  Anat.,  v, 

1906. 
R.  Mayer:  "Ueber  die  Bildung  des  Recessus  pharyngeus  medius  s.  Bursa  pharyngis 

in   zusammenhang   mit   der   Chorda   bei   menschlichen   Embryonen,"   Anat. 

Anzeiger,  xxxvii,  19 10. 
J.  F.  Meckel:  " Bildungsgeschichte  des  Darmkanals  der  Saugethiere  und  nament- 

lich  des  Menschen,"  Archiv  fiir  Anat.  und  Physiol.,  iii,  181 7. 
T.  MiRONESCu:  "Ueber  die  Entwicklung  der  Langerhans'schen  Inseln  bei  men- 
schlichen Embryonen,"  Arch,  fiir  mikrosk.  Anat.,  lxxvt,  191  i. 
H.  Moral:  "Ueber  die  ersten  Entwicklungsstadien  der  Glandula  Submaxillaris," 

Anat.  Hefte,  xlvii,  1913. 
H.  Moral:  "Ueber  die  ersten  Entwicklungsstadien  der  Glandula  Parotis,"  Anat. 

Hefte,  xlvii,  1913. 
E.  H.  Norris:  "The  early  morphogenesis  of  the  human  thyroid  gland."     Amer. 

Journ.  Anat.  xxiv,  1918. 
W.  J.  Otis:  "Die  Morphogenese  und  Histogenese  des  Analhockers  nebst  Bemerk- 


3l8  LITERATURE 

ungen  iiber  die  Entwicklung  der  Sphincter  ani  externus  beim  Menschen,"  Anat. 

Hefte,  XXX,  1906. 
J.  L.  Paulet:  "Kopf  und  bucconasale  Bildung  eines  menschlichen  Embryo  von 

14.7  mm.  Scheitelsteisslange,"  Archiv.f.  mikr.  Anat,,  lxxvi,  igio. 
R,  M.  Pearce:  "The  Development  of  the  Islands  of  Langerhans  in  the  Human 

Embryo,"  Amer.  Journ.  of  Anat.,  11,  1902. 
C.  Rose:  "Ueber  die  Entwicklung  der  Zahne  des    Menschen,  Archiv  fiir  mikrsok. 

Anat.,  XXXVIII,  1891. 
G.  Schorr:  "Zur  Entwickelungsgeschichte  des  secundaren  Gaumens,"  Anat.  Hefte, 

XXXVI,  T908. 
G.  Schorr:  "Ueber  Wolfsrachen  von  Standpunkt  der  Embryologie  und  patholog- 

ischen  Anatomie,"  Arch,  fiir  patholog.  Anat.,  cxcvii,  1909. 
H.  Sidier:  "Die  Entwicklung  des  secundaren  Gaumens  beim  Menschen."     Anat, 

Anzeiger,  xlvii,  191 5. 
A.  Swaen:  "Recherches  sur  le  developpement  du  foie,  du  tube  digestif,  de  I'arriere- 

cavit6  du  peritoine  et  du  mesentere,"  Journ.  de  I' Anat.  et  de  la  Physiol.,  xxxii, 

1896,  and  xxxiii,  1897. 
X  J.  Tandler:  "Zur  Entwickelungsgeschichte  des  menschlichen  Duodenum  in  friihen 

Embryonalstadien,"  Morphol.  Jahrbuch,  xxix,  1900. 
P.  Thompson:  "A  Note  on  the  Development  of  the  Septum  Transversum  and  the 

Liver,"  Journ.  Anat.  and  Phys.,  xlii,  1908. 
F.  W.  Thyng:  "Models  of  the  Pancreas  in  Embryos  of  the  Pig,  Rabbit,  Cat  and 

Man,"  Amer.  Journ.  Anat.,  vii,  1908. 
C.  ToLDT  AND  E.  Zuckerkandl:  "Ueber  die  Form  and  Texturveranderungen  der 

menschlichen  Leber  wahrend  des  Wachsthums,"  Sitzungsher.  der    kais.  Akad. 

Wissensch.  Wien.,  Math.-Naturwiss.  Classe,  lxxii,  1875. 
F.  Tourneux  and  p.  Verdun:  " Sur  les  premiers  developpements  de  la  Thyroide,  du 

Thymus  et  des  glandes  parathyroidiennes  chez  I'homme,"  Journ.  de  VAnat.  et 

de  la  Physiol.,  xxxiii,  1897.  ,, 

F.  Treves:  "Lectures  on  the  Anatomy  of  the  Intestinal  Canal  and  Peritoneum  in 

Man,"  British  Medical  Journal,  1,  1885. 


CHAPTER  XI 

THE  DEVELOPMENT  OF  THE  PERICARDIUM,  THE 
PLEUROPERITONEUM  AND  THE  DIAPHRAGM 

It  has  been  seen  (p.  230)  that  the  heart  makes  its  appearance  at 
a  stage  when  the  greater  portion  of  the  ventral  surface  of  the  intes- 
tine is  still  open  to  the  yolk-sac.  The  ventral  mesoderm  splits  to 
form  the  somatic  and  splanchnic  layers  and  the  heart  develops  as  a 
fold  in  the  latter  on  each  side  of  the  median  line,  projecting  into  the 
coelomic  cavity  enclosed  by  the  two  layers  (Fig.  138,  A).  As  the 
constriction  of  the  anterior  part  of  the  embryo  proceeds  the  two 
heart  folds  are  brought  nearer  together  and  later  meet,  so  that  the 
heart  becomes  a  cyHndrical  structure  lyiftg  in  the  median  line  of 
the  body  and  is  suspended  in  the  coelom  by  a  ventral  band,  the 
ventral  mesocardium,  composed  of  two  layers  of  splanchnic  meso- 
derm which  extend  to  it  from  the  ventral  wall  of  the  body,  and  by 
a  similar  band,  the  dorsal  mesocardium,  which  unites  it  with  the 
splanchnic  mesoderm  surrounding  the  digestive  tract.  The  ven- 
tral mesocardium  soon  disappears  (Fig.  138,  C)  and  the  dorsal  one 
also  vanishes  somewhat  later,  so  that  the  heart  comes  to  lie  freely 
in  the  coelomic  cavity,  except  for  the  connections  which  it  makes 
with  the  body-walls  by  the  vessels  which  enter  and  arise  from  it. 

The  coelomic  cavity  of  the  embryo  does  not  at  first  communi- 
cate with  the  extra-embryonic  coelom,  which  is  formed  at  a  very 
early  period  (see  p.  70),  but  later  when  the  splitting  of  the  embry- 
onic mesoderm  takes  place  the  two  cavities  become  directly 
continuous  behind  the  heart,  but  not  anteriorly,  since  the  ventral 
wall  of  the  body  is  formed  in  the  heart  region  before  the  union  can 
take  place.  It  is  possible,  therefore,  to  recognize  two  portions  in 
the  embryonic  coelom,  an  anterior  one,  the  parietal  cavity  (His), 
which  is  never  connected  laterally  with  the  extra-embryonic 
cavity,  and  a  posterior  one,  the  trunk  cavity,  which  is  so  connected. 

319 


320 


THE    PERICARDIUM   AND   PLEURO-PERITONEUM 


The  heart  is  situated  in    the    parietal  cavity,  a    considerable 
portion  of  which  is  destined  to  become  the  pericardial  cavity. 

Since  the  parietal  cavity  lies  immediately  anterior  to  the  still 
wide  yolk-stalk,  as  may  be  seen  from  the  position  of  the  heart  in 
the  embryo  shown  in  Fig.  54,  it  is  bounded  posteriorly  by  the 

yolk-stalk.  This  boundary  is  com- 
plete, however,  only  in  the  median 
line,  the  cavity  being  continuous  on 
either  side  of  the  yolk-stalk  with  the 
trunk-cavity  by  passages  which  have 
been  termed  the  recessus  parietales 
(Fig.  196,  Rca).  Passing  forward 
toward  the  heart  in  the  splanchnic 
mesoderm  which  surrounds  the  yolk- 
stalk  are  the  large  vitelline  veins, 
one  on  either  side,  and  these  shortly 
become  so  large  as  to  bring  the 
splanchnic  mesoderm  in  which  they 
lie  in  contact  with  the  somatic 
mesoderm  which  forms  the  lateral 
walls  of  each  recess.  Fusion  of  the 
two  layers  of  mesoderm  along  the 
course  of  the  veins  now  takes  place, 
and  each  recess  thus  becomes  di- 
vided into  two  parallel  passages, 
which  have  been  termed  the  dorsal 
(Fig.  197,  rpd)  and  ventral  {rpv) 
parietal  recesses.  Later  the  two 
veins  fuse  in  the  upper  portion  of 
their  course  to  form  the  beginning  of 
the  sinus  venosus,  with  the  result  that  the  ventral  recesses  become 
closed  below  and  their  continuity  with  the  trunk-cavity  is  inter- 
rupted, so  that  they  form  two  blind  pouches  extending  downward 
a  short  distance  from  the  ventral  portion  of  the  floor  of  the  parietal 
cavity.  The  dorsal  recesses,  however,  retain  their  continuity  with 
the  trunk-cavity  until  a  much  later  period. 


Om 


Rca 

Fig.  196,  —  Reconstruction 
OF  A  Rabbit  Embryo  of  Eight 
Days,  with  the  Pericardial 
Cavity  Laid  Open. 

A,  Auricle;  Aoh,  aortic  bulb;  A, 
v.,  atrio- ventricular  communica- 
tion;'Om,  vitelline  vein;  Pc,  peri- 
cardial cavity;  Rca,  parietal  recess; 
Sv,  sinus  venosus;  V,  ventricle. — 
(.His.) 


THE   PERICARDIUM   AND    PLEURO-PERITONEUM 


321 


By  the  fusion  of  the  vitelline  veins  mentioned  above,  there  is 
formed  a  thick  semilunar  fold  which  projects  horizontally  into  the. 
coelom  from  the  ventral  wall  of  the  body  and  forms  the  floor  of  the 
ventral  part  of  the  parietal  recess.  This  is  known  as  the  septum 
transversum,  and  besides  containing  the  anterior  portions  of  the 
vitelline  veins,  it  also  furnishes  a  passage  by  which  the  ductus 
Cuvieri,  formed  by  the  union  of  the  jugular  and  cardinal  veins, 
reach  the  heart.  Its  dorsal  edge  is  continuous  in  the  median  line 
with  the  mesoderm  surrounding  the  digestive  tract  just  opposite 
the  region  where  the  hver  outgrowth  will  form,  but  laterally  this 
edge  is  free  and  forms  the  ventral  walls  of  the  dorsal  parietal 
recess.     An  idea  of  the  relations  of  the  septum  at  this  stage  may  be 


yom 


rpv 


Fig.  197. — Transverse  Sections  of  a  Rabbit  Embryo  showing  the  Division  of 

THE  Parietal  Recesses  by  the  Vitelline  Veins. 

am.  Amnion;  rp,  parietal  recess;  rpd  and  rpv,  dorsal  and  ventral  divisions  of  the 

parietal  recess;  vom,  vitelline  vein. — (Ravn.) 

obtained  from  Fig.  198,  which  represents  the  anterior  surface  of  the 
septum,  together  with  the  related  parts,  in  a  rabbit  embryo  of 
nine  days. 

The  Separation  of  the  Pericardial  Cavity. — The  septum  trans- 
versum is  at  first  almost  horizontal,  but  later  it  becomes  decidedly 
oblique  in  position,  a  change  associated  with  the  backward  move- 
ment of  the  heart.  As  the  closure  of  the  ventral  wall  of  the  body 
extends  posteriorly  the  ventral  edge  of  the  septum  gradually  slips 
downward  upon  it,  while  the  dorsal  edge  is  held  in  its  former  posi- 
tion by  its  attachment  to  the  wall  of  the  digestive  tract  and  the 

ductus  Cuvieri.     The  anterior  surface  of  the  septum  thus  comes 
21 


322 


THE   PERICARDIUM   AND    PLEURO-PERITONEUM 


to  look  ventrally  as  well  as  forward,  and  the  parietal  cavity,  having 
taken  up  into  itself  the  blind  pouches  which  represented  the 
ventral  recesses,  comes  to  lie  to  a  large  extent  ventral  to  the  poste- 
rior recesses.  As  may  be  seen  from  Fig.  198,  the  ductus  Cuvieri, 
as  they  bend  from  the  lateral  walls  of  the  body  into  the  free  edges 
of  the  septum,  form  a  marked  projection  which  diminishes  con- 
siderably the  opening  of  the  dorsal  recesses  into  the  parietal  cavity. 


am 


Fig.  198. — Reconstruction  from  a  Rabbit  Embryo  of  Nine  Days  showing  the 

Septum  Transversum  from  Above. 
am.  Amnion;  at,  atrium;  dc,  ductus  Cuvieri ;V^(/,  dorsal  parietal  recess. — (Ravn.) 

In  later  stages  this  projection  increases  and  from  its  dorsal  edge  a 
fold,  which  may  be  regarded  as  a  continuation  of  the  free  edge  of 
the  septum,  projects  into  the  upper  portions  of  the  recesses  and 
eventually  fuses  with  the  median  portion  of  the  septum  attached 
to  the  wall  of  the  gut.  In  this  way  the  parietal  cavity  becomes  a 
completely  closed  sac,  and  is  henceforward  known  as  the  pericar- 
dial cavity,  the  original  coelom  being  now  divided  into  two  portions, 
(i)  the  pericardial,  and  (2)  the  pleuro peritoneal  cavities,  the  latter 


THE   DIAPHRAGM 


323 


consisting  of  the  abdominal  coelom  together  with  the  two  dorsal 
parietal  recesses  which  have  been  separated  from  the  pericardia  1_ 
(parietal)  cavity  and  are  destined  to  be  converted  into  the  pleural 
cavities. 

The  Formation  of  the  Diaphragm. — It  is  to  be  remembered  that 
the  attachment  of  the  transverse  septum  to  the  ventral  wall  of  the 
digestive  tract  is  opposite  the  point  where  the  liver  outgrowth 
develops.  When,  therefore,  the  outgrowth  appears,  it  pushes  its 
way  into  the  substance  of  the  septum,  which  thus  acquires  a  very 
considerable  thickness,  especially  toward  its  dorsal  edge,  and  it 
furthermore  becom,es  differentiated  into  two  layers,  an  upper  one, 


Fig.  199. — Diagrams  of  (A)  a  Sagittal  Section  of  an  Embryo  showing  the 
Liver  Enclosed  within  the  Septum  Transversum;  {B)  a  Frontal  Section  of 
THE  Same;  (C)  a  Frontal  Section  of  a  Later  Stage  when  the  Liver  has  Sepa- 
rated FROM  THE  Diaphragm. 

All,  Allantois;  CI,  cloaca;  D,  diaphragm;  Li,  liver;  Ls,  falciform  ligament  of  the 
liver;  M,  mesentery;  Mg,  mesogastrium;  Pc,  pericardium;  S,  stomach;  ST,  septum 
transversum;  U,  umbilicus. 


which  forms  the  floor  of  the  ventral  portion  of  the  pericardial 
cavity  and  encloses  the  Cuvierian  ducts,  and  a  lower  one  which 
contains  the  liver.  The  upper  layer  is  comparatively  thin,  while 
the  lower  forms  the  greater  part  of  the  thickness  of  the  septum, 
its  posterior  surface  meeting  the  ventral  wall  of  the  abdomen  at 
the  level  of  the  anterior  margin  of  the  umbilicus  (Fig.  199,  A). 

In  later  stages  of  development  the  layer  containing  the  liver 
becomes  separated  from  the  upper  layer  by  two  grooves  which, 


324  THE   DIAPHRAGM 

appearing  at  the  sides  and  ventrally  immediately  over  the  liver 
(Fig.  199,  B),  gradually  deepen  toward  the  median  line  and  dor- 
sally.  These  grooves  do  not,  however,  quite  reach  the  median 
line,  a  portion  of  the  lower  layer  of  the  septum  being  left  in  this 
region  as  a  fold,  situated  in  the  sagittal  plane  of  the  body  and 
attached  above  to  the  posterior  surface  of  the  upper  layer  and 
below  to  the  anterior  surface  of  the  liver,  beyond  which  it  is  con- 
tinued down  the  ventral  wall  of  the  abdomen  to  the  umbilicus 
(Fig.  199,  C,  Ls).  This  is  the  falciform  ligament  of  the  liver  of 
adult  anatomy,  and  in  the  free  edge  of  its  prolongation  down  the 
ventral  wall  of  the  abdomen  the  umbilical  vein  passes  to  the  under 
surface  of  the  liver,  while  the  free  edge  of  that  portion  which  lies 
between  the  liver  and  the  digestive  tract  contains  the  vitelline 
(portal)  vein,  the  common  bile-duct,  and  the  hepatic  artery.  The 
diagram  given  in  Fig.  199  will,  it  is  hoped,  make  clear  the  mode  of 
formation  and  the  relation  of  this  fold,  which,  in  its  entirety,  con- 
stitutes what  is  sometimes  termed  the  ventral  mesentery. 

And  not  only  do  the  grooves  fail  to  unite  in  the  median  line,  but 
they  also  fail  to  completely  separate  the  liver  from  the  upper  layer 
of  the  septum  dorsally,  the  portion  of  the  lower  layer  which  persists 
in  this  region  forming  the  coronary  ligament  of  the  hver.  The 
portion  of  the  lower  layer  which  forms  the  roof  of  the  grooves  be- 
comes the  layer  of  peritoneum  covering  the  posterior  surface  of  the 
upper  layer  (which  represents  the  diaphragm),  while  the  portion 
which  remains  connected  with  the  liver  constitutes  its  peritoneal 
investment. 

In  the  meantime  changes  have  been  taking  place  in  the  upper 
layer  of  the  septum.  As  the  rotation  of  the  heart  occurs,  so  that 
its  atrial  portion  comes  to  lie  anterior  to  the  ventricle,  the  Cuvier- 
ian  ducts  are  drawn  away  from  the  septum  and  penetrate  the  pos- 
terior wall  of  the  pericardium,  the  separation  being  assisted  by 
the  continued  descent  of  the  attachment  of  the  edge  of  the  septum 
to  the  ventral  wall  of  the  body.  During  the  descent,  when  the 
upper  layer  of  the  septum  has  reached  the  level  of  the  fourth  cer- 
vical segment,  portions  of  the  myotomes  of  that  segment  become 
prolonged  into  it  and  the  layer  assumes  the  characteristics  of  the 


THE   PLEURA  325 

diaphragm,  the  supply  of  whose  musculature  from  the  fourth 
cervical  nerves  is  thus  explained. 

The  PleurcB. — The  diaphragm  is  as  yet,  however,  incomplete 
dorsally,  where  the  dorsal  parietal  recesses  are  still  in  continuity 
with  the  trunk-cavity.  With  the  increase  in  thickness  of  the 
septum  transversum,  these  recesses  have  acquired  a  considerable 
length  antero-posteriorly,  and  into  their  upper  portions  the  out- 
growths from  the  lower  part  of  the  pharynx  which  form  the  lungs 
(see  p.  334)  begin  to  project.  The  recesses  thus  become  trans- 
formed into  the  pleural  cavities,  and  as  the  diaphragm  continues 
to  descend,  slipping  down  the  ventral  wall  of  the  body  and  drawing 
with  it  the  pericardial  cavity,  the  latter  comes  to  He  entirely 
ventral  to  the  pleural  cavities.  The  free  borders  of  the  diaphragm, 
which  now  form  the  ventral  boundaries  of  the  openings  by  which 
the  pleural  and  peritoneal  cavities  communicate,  begin  to  approach 
the  dorsal  wall  of  the  body,  with  which  they  finally  unite  and  so 
complete  the  separation  of  the  cavities.  The  pleural  cavities 
continue  to  enlarge  after  their  separation  and,  extending  laterally, 
pass  between  the  pericardium  and  the  lateral  walls  of  the  body  until 
they  finally  almost  completely  surround  the  pericardium.  The  in- 
tervals between  the  two  pleurae  form  what  are  termed  the  mediastina. 

The  downward  movement  of  the  septum  transversum  extends 
through  a  very  considerable  interval,  which  may  be  appreciated 
from  the  diagram  shown  in  Fig.  200.  From  this  it  may  be  seen 
that  in  early  embryos  the  septum  is  situated  just  in  front  of  the 
first  cervical  segment  and  that  it  lies  very  obliquely,  its  free  edge 
being  decidedly  posterior  to  its  ventral  attachment.  When  the 
down\^ard  displacement  occurs,  the  ventral  edge  at  first  moves  more 
rapidly  than  the  dorsal,  and  soon  comes  to  lie  at  a  much  lower 
level.  The  downward  movement  continues  throughout  the  entire 
length  of  the  cervical  and  thoracic  regions,  and  when  the  level  of 
the  tenth  thoracic  segment  is  reached  the  separation  of  the  pleural 
and  peritoneal  cavities  is  completed,  and  then  the  dorsal  edge  ^ 
begins  to  descend  more  rapidly  than  the  ventral,  so  that  the 
diaphragm  again  becomes  oblique  in  the  same  sense  as  in  the 
beginning,  a  position  which  it  retains  in  the  adult. 


326 


THE    PERITONEUM 


t«<^U/ 


OU/tnit£ 


DaUai 


The  Development  of  the  Peritoneum. — -The  peritoneal  cavity  is 
developed  from  the  trunk-cavity  of  early  stages  and  is  at  first  in 
free  communication  on  all  sides  of  the  yolk-stalk  with  the  extra- 
embryonic coelom.  As  the  ventral  wall  of  the  body  develops  the 
two  cavities  become  more  and  more  separated,  and  with  the 
formation  of  the  umbilical  cord  the  separation  is  complete.  Along 
the  mid-dorsal  line  of  the  body  the  archenteron  forms  a  projection 
into  the  cavity  and  later  moves  further  out  from  the  body-wall 

into  the  cavity,  pushing  in  front  of 
it  the  peritoneum,  which  thus  comes  to 
surround  the  intestine,  forming  its  serous 
coat,  and  from  it  is  continued  back  to 
the  dorsal  body-wall  forming  the 
mesentery. 

It  has  already  been  seen  that  on  the 
separation  of  the  liver  from  the  septum 
transversum,  the  tissue  of  the  latter 
gives  rise  to  the  peritoneal  covering  of 
the  liver  and  of  the  posterior  surface 
of  the  diaphragm,  and  also  to  the 
ventral  mesentery.  When  the  separ- 
ation is  taking  place,  the  rotation  of 
the  stomach  already  described  ( p.  303) 
occurs,  with  the  result  that  the  portion 
of  the  ventral  mesentery  which 
stretches  between  the  lesser  curvature 
of  the  stomach  and  the  liver  shares  in 
the  rotation  and  comes  to  lie  in  p,  plane 
practically  at  right  angles  with  that 
of  the  falciform  ligament,  its  surfaces  looking  dorsally  and 
ventrally  and  its  free  edge  being  directed  toward  the  right. 
This  portion  of  the  ventral  mesentery  forms  what  is  termed 
the  lesser  omentum,  and  between  it  and  the  dorsal  surface 
of  the  stomach  as  the  ventral  boundaries,  and  the  dorsal  wall 
of  the  abdominal  cavity  dorsally,  there  is  a  cavity,  whose  floor 
is  formed  by  the  dorsal  mesentery  of  the  stomach,  the  meso- 


FiG.  200. — Diagram  show 
iNG  THE  Position  of  the  Dia 
PHRAGM  IN  Embryos  of  Dif 
FERENT  Ages. — {Mall.) 


THE    PERITONEUM  327 

gastrium,  the  roof  by  the  under  surface  of  the  left  half  of  the  liver, 
while  to  the  right  it  communicates  with  the  general  peritoneal- 
cavity  dorsal  to  the  free  edge  of  the  lesser  omentum.  This  cavity 
is  known  as  the  bursa  omentalis  (lesser  sac  of  the  peritoneum) ,  and 
the  opening  into  it  from  the  general  cavity  or  greater  sac  is  termed 
\h&  epiploic  foramen  (foramen  of  Winslow).  Later,  the  floor  of 
the  lesser  sac  is  drawn  downward  to  form  a  broad  sheet  of  peri- 
toneum lying  ventral  to  the  coils  of  the  small  intestine  and  con- 
sisting of  four  layers;  this  represents  the  great  omentum  of  adult 
anatomy  (Fig.  204). 

Although  the  form  assumed  by  the  bursa  omentalis  is  asso- 
ciated with  the  rotation  of  the  stomach,  it  seems  probable  that 
its  real  origin  is  independent  of  that  process  (Broman).  The 
subserous  tissue  of  the  transverse  septum  is  at  first  thick  and  in- 
cludes not  only  the  hver,  but  also  the  pancreas  and  the  portion 
of  the  digestive  tract  which  becomes  the  stomach  and  the  upper 
part  of  the  duodenum  (Fig.  199,  A).  The  shrinkage  of  this 
tissue,  by  which  these  organs  become  separated  from  the  septum, 
cannot  take  place  evenly  on  account  of  the  relations  which  the 
organs  bear  to  one  another,  so  that  on  the  right  side  certain 
peritoneal  recesses  are  formed,  one  between  the  right  lung  and 
the  stomach,  a  second  between  the  liver  and  the  stomach,  and  a 
third  between  the  pancreas  and  the  same  structure.  In  man 
these  three  recesses  communicate  with  one  another  to  form  the 
primary  bursa  omentalis,  and  open  by  a  common  epiploic  foramen 
into  the  general  peritoneal  cavity.  The  rotation  of  the  stomach, 
which  takes  place  later,  merely  serves  to  modify  the  original 
bursa. 

In  the  human  embryo  a  small  recess  also  forms  upon  the  left  side 
between  the  left  lung  and  the  stomach.  Later  it  separates  from  the 
rest  of  the  bursa  omentalis  and  passes  up  along  the  side  of  the  oesoph- 
agus, coming  to  lie  on  its  right  side  between  it  and  the  diaphragm. 
It  gives  rise  to  a  small  serous  sac  that  lies  beneath  the  infracardial 
lobe  of  the  right  lung,  when  this  is  present,  and  hence  has  been  termed 
the  infracardial  bursa. 

Below  the  level  of  the  upper  part  of  the  duodenum  the  ventral 
mensentery  is  wanting;  only  the  dorsal  mesentery  occurs.     So 


328 


THE   PERITONEUM 


long  as  the  intestine  is  a  straight  tube  the  length  of  the  intestinal 
edge  of  this  mesentery  is  practically  equal  to  that  of  its  dorsal 
attached  edge.  The  intestine,  however,  increasing  in  length 
much  more  rapidly  than  the  abdominal  walls,  the  intestinal  edge 
of  the  mesentery  soon  becomes  very  much  longer  than  the  at- 
tached edge,  and  when  the  intestine  grows  out  into  the  umbiHcal 
coelom  the  mesentery  accompanies  it  (Fig.  201).  As  the  coils  of 
the  intestine  develop,  the  intestinal  edge  of  the  mesentery  is 
thrown  into  corresponding  folds,  and  on  the  return  of  the  intestine 
to  the  abdominal  cavity  the  mesentery  is 
thrown  into  a  somewhat  funnel-like  form  by 
the  twisting  of  the  intestine  to  form  its  primary 
loop  (Fig.  202).  All  that  portion  of  the  mes- 
entery which  is  attached  to  the  part  of  the 
intestine  which  will  later  become  the  jejunum, 
ileum,  ascending  and  transverse  colon,  is  at- 
tached to  the  body-wall  at  the  apex  of  the 
funnel,  at  a  point  which  lies  to  the  left  of  the 
duodenum. 

Up  to  this  stage  or  to  about  the  middle  of 
the  fourth  month  the  mesentery  has  retained 
its  attachment  to  the  median  line  of  the 
dorsal  wall  of  the  abdomen  throughout  its 
entire  length,  but,  later,  fusions  of  certain 
whereby  the  original  con- 
dition is  greatly  modified.  One  of  the  earl- 
iest of  these  fusions  results  in  the  formation 
of  the  transverse  mesocolon,  whose  attach- 
ment to  the  dorsal  wall  of  the  abdomen  is  at  right  angles  to  the 
original  line  of  attachment  of  the  mesentery.  This  condition  is 
brought  about  in  the  following  manner.  By  the  twisting  of  the 
primary  loop  of 'the  intestine  the  ascending  and  descending  colons 
are  brought  into'^apposition  with  the  lateral  walls  of  the  abdominal 
cavity,  while  the  transverse  colon,  since  it  passes  ventral  to  the 
duodenum,  has  a  more  ventral  position  (Fig.  202).  The  right 
layer  of  the  portion  of  the  mesentery  attached  to  the  ascending 


Fig.  201. — Diagram 
showing  the  arrange- 
ment of  the  m  esentery 
AND  Visceral  Branches 
OF  THE  Abdominal  portions  OCCUr 
Aorta  in  an  Embryo 
OF  Six  Weeks. 
p.  Pancreas;  S,  stomach; 
Sp,  spleen.— (ToW/.) 


THE    PERITONEUM 


329 


colon  is  thus  brought  into  contact  with  the  parietal  peritoneum 
lining  the  right  wall  of  the  abdomen  and  fuses  with  it  and,  sinii- 
larly,  the  left  layer  of  the  descending  mesocolon  fuses  with  the 
parietal  peritoneum  of  the  left  wall  of  the  abdomen,  as  is  shown 
in  Fig.  204.  The  ascending  and  descending  colons  thus  lose  their 
mesentery  and  become  permanently  fixed  in  position,  but  since 
the  transverse  mesocolon  does  not  come  into  contact  with  the 
parietal  peritoneum,  it  remains  distinct  and  has  acquired  a  trans- 
verse line  of  attachment  to  the  body  wall. 


Fig.  202. — Diagrams  Illustrating  the  Development  of  the  Great  Omentum 
AND  THE  Transverse  Mesocolon. 
bid.  Caecum;  dd,  small  intestine;  dg,  yolk-stalk;  di,  colon;  du  duodenum;  gc, 
greater  curvature  of  stomach;  gg,  bile  duct;  gn,  mesogastrium;  k,  point  where  the 
loops  of  the  intestine  cross;  mc,  mesocolon;  md,  rectum;  mes,  mesentery;  wf,  vermi- 
form appendix. — {Her  twig.) 


This  hne  of  attachment  passes  across  the  duodenum  and  forces 
this  portion  of  the  intestine  against  the  dorsal  abdominal  wall, 
with  the  result  that  its  dorsal  mesentery  disappears,  becoming  con- 
verted into  subserous  areolar  tissue.  The  duodenum  and  pan- 
creas, which  latter  is  essentially  an  outgrowth  from  the  duodenum 
into  the  dorsal  mesentery,  thus  assume  the  position  which  char- 
acterizes them  in  the  adult,  becoming  retroperitoneal  and  lying 
behind  the  root  of  the  transverse  mesocolon. 


330 


THE    PERITONEUM 


The  fusion  of  the  mesentery  of  the  ascending  and  descending  colon 
remains  incomplete  in  a  considerable  number  of  cases  (one-fourth  to 
one- third  of  all  cases  examined),  and  in  these  the  colons  are  not  perfectly 
fixed  to  the  abdominal  wall.  It  may  also  be  pointed  out  that  the  caecum 
and  appendix,  being  primarily  a  lateral  outpouching  of  the  intestine, 
do  not  possess  any  true  mesentery,  but  are  completely  enclosed  by 
peritoneum.  Usually  a  falciform  fold  of  peritoneum  may  be  found 
extending  along  one  surface  of  the  appendix  to  become  continuous 
with  the  left  layer  of  the  mesentery  of  the  ileum.  This,  however,  is 
not  a  true  mesentery,  and  is  better  spoken  of  as  a  mesenteriole. 


<'"''•"'"  . 


Fig.  203. — Diagrams  Illustrating  the  Manner  in  Which  the  Fixation  of 
THE  Descending  Colon   (C)  takes  Place. 

One  other  fusion  is  still  necessary  before  the  adult  condition  of 
the  mesentery  is  acquired.  The  great  omentum  consists  of  two 
folds  of  peritoneum  which  start  from  the  greater  curvature  of  the 
stomach  and  pass  downward  to  be  reflected  up  again  to  the  dorsal 
wall  of  the  abdomen,  which  they  reach  just  anterior  to  (above) 
the  line  of  attachment  of  the  transverse  mesocolon  (Fig.  204  A). 
At  first  the  attachment  of  the  omentum  is  vertical,  since  it  repre- 
sents the  mesogastrium,  but  later,  by  fusion  with  the  parietal 
peritoneum,  it  assumed  a  transverse  direction.  By  this  change 
the  lower  layer  of  the  omentum  is  brought  in  contact  with  the 
upper  layer  of  the  transverse  mesocolon  and  a  fusion  and  de- 
generation of  the  two  results  (Fig.  204  5),  a  condition  which  brings 
it  about  that  the  omentum  seems  to  be  attached  to  the  transverse 
colon.     The  transverse  mesocolon,  as  it  occurs  in  the  adult,  really 


THE    PERITONEUM 


33'^ 


consists  partly  of  a  portion  of  the  original  transverse  mesocolon 
and  partly  of  a  layer  of  the  great  omentum.  _  _ 

By  these  various  changes  the  line  of  attachment  of  the  mesen- 
tery to  the  dorsal  wall  of  the  body  has  become  somewhat  compli- 
cated and  has  departed  to  a  very  considerable  extent  from  its 
original  simple  vertical  arrangement.     If  all  the  viscera  be  re- 


FiG.  204. — Diagrams  showing  the* Development  of  the  Great  Omentum  and 

ITS  Fusion  with  the  Transverse  Mesocolon. 

B,  bladder;  c,  transverse  colon;  d,  duodenum;  Li,  liver;  p,  pancreas;  2?,  rectum;  S. 

stomach;  U.  uterus. — (After  Allen  Thomson.) 

moved  from  the  body  of  an  adult  and  the  mesentery  be  cut  close 
to  the  hne  of  its  attachment,  the  course  of  the  latter  will  be  seen 
to  be  as  follows  (Fig.  205) :  Descending  from  the  under  surface  of 
the  diaphragm  are  the  lines  of  attachment  of  the  falciform  liga- 
ment, which  on  reaching  the  liver  spread  out  to  become  the  coro- 
nary and  lateral  ligaments  of  that  organ,  all  these  structures  being 
derivatives  of  the  ventral  mesentery.  A  little  to  the  left  of  the 
median  plane  these  lines  become  continuous  with  those  of  the 
mesogastrium  which  curve  downward  toward  the  left  and  are 


332 


THE    PERITONEUM 


continued  into  the  transverse  lines  of  the  transverse  mesocolon. 
Between  these  last,  in  a  slight  prolongation,  there  may  be  seen 
to  the  right  the  cut  end  of  the  first  portion  of  the  duodenum  as  it 
passes  back  to  the  dorsal  wall  of  the  abdomen,  and  at  about  the 


Fig.  205. — The  Lines  of  Reflection  of  the  Peritoneum  from  the  Abdom- 
inal Wall  to  the  Various  Organs. — {From  Morris'  Anatomy.) 
ac.  Area  of  attachment  of  ascending  colon;  BO,  bursa  omentalis;  cor.  I.,  coronary- 
ligament;  d.c,  area  of  attachment  of  descending  colon;  Du,  duodenuna;  EF,  epiploic 
foramen;  FL,  falciform  ligament;  mg,  mesogastrium;  mes,  mesentery  of  small  intes- 
tine; ees,  oesophagus;  R,  rectum;  Sc,  sigmoid  mesocolon;  tr.  I.,  left  triangular  liga- 
ment; tr.mc,  transverse  mesocolon;  v.c.i,  inferior  vena  cava. 

mid-dorsal  line  the  cut  end  of  its  last  part  becomes  visible  as  it 
passes  ventrally  again  to  become  the  jejunum.     From  the  trans- 


LITERATURE  333 

verse  mesocolon  three  lines  of  attachment  pass  downward;  the 
two  lateral  broad  ones  represent  the  lines  of  fixation  of  the  as- 
cending and  descending  colons,  while  the  narrower  median  one 
which  curves  to  the  right,  represents  the  attachment  of  the 
mesentery  of  the  small  intestine  other  than  the  duodenum.  Fin- 
ally, from  the  lower  end  of  the  fixation  line  of  the  descending  colon 
the  mesentery  of  the  sigmoid  colon  is  continued  downward. 

The  special  developments  of  the  peritoneum  in  connection 
with  the  genito-urinary  apparatus  will  be  considered  in  Chapter 
XIII. 

LITERATURE 

I.   Broman:  "Ueber  die  Entwicklung  und  Bedeutung  der  Mesenterien  und  der 

Korpenhohlen  bei  den  Wirbeltieren,"  Ergehn.  der  Anat.  u.  Entw.,  xv,  1906. 
A.  Bracket:  "Die  Entwdckelung  der  grossen  Korperhohlen  und  ihre  Trennung  von 

Einander,"  Ergehnisse  der  Anat.  und  Entwickelungsgesch.,  vii,  1898. 
W.  His:  "Mittheilungen  zur  Embryologie  der  Saugethiere  und  des  Menschen," 

Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  1881. 
F.  P.  Mall:  "Development  of  the  Human  Coelom,"  Journat-of  Morphol.,  xii,  1897. 
F.  P.  Mall:  "On  the  Development  of  the  Human  Diaphragm,"  Johns  Hopkins 

Hospital  Bull.,  xii,  1901. 

E.  Ravn:  "Ueber  die  Bildung  der  Scheidewand  zwischen  Brust-und  Bauchhohle  in 

Saugethierembryonen,"  Archiv  fiir  Anat.  und  Physiol.,  Anat.  Ahlh.,  1889. 
A.  Swaen:  "Recherches  sur  le  d6veloppement  du  foie,  du  tube  digestif,  de  I'arriere- 

cavit6  du  peritoine  et  du  mesentere,"  Journ.  de  I' Anat.  et  de  la  Physiol.,  xxxii, 

1896;  XXXIII,  1897. 
C.  Toldt:  "Bau  und  Wachstumsveranderungen  der  Gekrose  des  menschlichen 

Darmkanals,"  Denkschr.  der  kais.  Akad.  Wissensch.  Wien,  Math.-Naturwiss. 

Classe,  xli,  1879. 
C.  Toldt:  "Die  Darmgekrose  und  Netze  im  gesetzmassigen  und  gestezwidrigen 

Zustand,"  Denkschr.  der  kais.  Akad.  Wissensch.  Wien.  Math.-Naturwiss.  Classe, 

LVI,  1889. 

F.  Treves:  "Lectures  on  the  Anatomy  of  the  Intestinal  Canal  and  Peritoneum," 

British  Medical  Journal,  i,  1885. 


CHAPTER  XII 


THE  DEVELOPMENT  OF  THE  ORGANS  OF  RESPIRATION 


RP 


The  Development  of  the  Lungs.— The  first  indication  of  the 
lungs  and  trachea  is  found  in  embryos  of  about  3.2  mm.  in  the 
form  of  a  groove  on  the  ventral  surface  of  the  oesophagus,  at  first 
extending  almost  the  entire  length  of  that  portion  of  the  digestive 
tract.  As  the  oesophagus  lengthens  the  lung  groove  remains  con- 
nected with  its  upper  portion 
(Fig.  184  A)^  and  furrows  which 
appear  along  the  line  of  junction 
of  the  groove  and  the  oesophagus 
gradually  deepen  and  separate 
the  two  structures  (Fig.  184,  B). 
The  separation  takes  place  earUest 
at  the  lower  end  of  the  groove  and 
thence  extends  upward,  so  that 
the  groove  is  transformed  into  a 
Pig.  I06.-P0RT10N  OF  A  Section  cylindrical  pouch  lying  ventral  to 
THROUGH  AN  Embryo  OF  THE  FouRTH  thc  ocsophagus  aud  dorsal  to  the 
^T  Aorta;  DC,  ductus  Cuvieri;  L,   heart  and  Opening  with  the  oeso- 

lung;    O.  oesophagus;    2?P,  parietal  re-    phagUS  iutO  the  terminal portioU  of 
cess;    VOm,  vitelline  vein. — (Toldt.) 

the  pharynx. 

Soon  after  the  separation  of  the  groove  from  the  oesophagus 
its  lower  end  becomes  enlarged  and  bilobed,  and  since  this  lower 
end  Hes,  with  the  oesophagus,  in  the  median  attached  portion  of 
the  dorsal  edge  of  the  septum  transversum,  the  lobes,  as  they  en- 
large, project  into  the  dorsal  parietal  recesses  (Fig.  206),  and  so  be- 
come surrounded  by  the  peritoneal  lining  of  the  recesses,  which 
later  become  the  pleural  cavities. 

The  lobes,  which  represent  the  lungs,  do  not  long  remain 
simple,  but  bud-like  processes  arise  from  their  cavities,  three  ap- 

334 


THE   LUNGS 


335 


pearing  in  the  right  lobe  and  two  in  the  left  (Fig.  207,  A),  and  as 
these  increase  in  size  and  give  rise  to  additional  outgrowths,  the 
structure  of  the  lobes  rapidly  becomes  complicated  (Fig.  207,  B 
and  C) . 

The  lower  primary  process  on  each  side  may  be  regarded  as  a 
prolongation  of  the  bronchus,  while  the  remaining  process  or 
processes  represent  lateral  outgrowths  from  it.  Considerable 
difference  of  opinion  has  existed  as  to  the  nature  of  the  further 
branching  of  the  bronchi,  some  authors  regarding  it  as  a  succession 


-'-.ip 


c^. 


B  C 

Fig.  207. — Reconstruction  of  the  Lung  Outgrowths  of  Embryos  of  (A)  4,3,  (B) 

8.5,   AND   (C)    10.5   MM. 

Ap,  Pulmonary  artery;  Ep,  eparterial  bronchus;  Vp,  pulmonary  vein;  /,    seoond 
lateral  bronchus;  II,  main  bronchi. — (His.) 

of  dichotomies,  one  branch  of  each  of  these  placing  itself  so  as  to 
be  in  the  line  of  the  original  main  bronchus,  while  the  other  comes 
to  resemble  a  lateral  outgrowth,  and  other  observers  have  held 
that  the  main  bronchus  has  an  uninterrupted  growth,  all  other 
branches  being  lateral  outgrowths  from  it,  and  the  branching 
therefore  a  monopodial  process.  The  recent  thorough  study  by 
Flint  of  thexievelopment  of  the  lung  of  the  pig  shows  that,  in  that 
form  at  least,  the  branching  is  a  monopodial  one,  and  that  from 
the  main  bronchus  as  it  elongates  four  sets  of  secondary  growths 
develop,  namely,  a  strong  lateral,  a  dorsal,  a  ventral,  and  a  weak 
and  variable  medial  set. 

There  is  a  general  tendency  for  the  individual  branches  of  the 


336 


THE   LUNGS 


various  sets  to  be  arranged  in  regular  succession  and  for  their 
development  to  be  symmetrical  in  the  two  lungs.  But  on  ac- 
count of  the  necessity  under  which  the  lungs  are  placed  of  adapt- 
ing themselves  to  the  neighboring  structures  and  at  the  same  time 
affording  a  respiratory  surface  as  large  as  possible,  an  amount  of 
asymmetry  supervenes.  Thus,  it  has  already  been  noted  that  in 
the  earliest  branching  a  single  lateral  bronchus  is  formed  in  the 

left  lung  and  two  in  the  right.  The 
uppermost  of  these  latter,  the  first 
lateral  bronchus,  is  unrepresented  in 
the  left  lung,  and  is  peculiar  in  that  it 
lies  behind  the  right  pulmonary  artery 
(Fig.  207,  C),  or  in  the  adult,  after  the 
recession  of  the  heart,  above  it,  whence 
it  is  termed  the  eparterial  bronchus.  Its 
absence  on  the  left  side  is  perhaps  due  to 
its  suppression  to  permit  the  normal 
recession  of  the  aortic  arch  (Flint). 

So,  too,  the  inclination  of  the  heart 
causes  a  suppression  of  the  second 
ventral  bronchus  in  the  left  lung,  but 
at  the  same  time  it  affords  opportunity 
for  an  excessive  development  of  the 
corresponding  bronchus  of  the  right 
lung,  which  pushes  its  way  between  the 
heart  and  the  diaphragm  and  is  known 
as  the  infracardiac  bronchus. 
As  soon  as  the  unpaired  first  lateral  bronchus  and  the  paired 
second  lateral  bronchi  are  formed  mesenchyme  begins  to  collect 
around  each  of  them  and  also  around  the  main  bronchi,  the  lobes 
of  the  adult  lung,  three  in  the  right  lung  and  two  in  the  left,  being 
thus  outHned.  A  development  of  mesenchyme  also  takes  place 
around  the  excessively  developed  right  second  ventral  bronchus, 
and  sometimes  produces  a  well-marked  infra-cardiac  lobe  in  the 
right  lung. 

In  later  stages  the  various  bronchi  of  each  lobe  give  rise  to 


Pig.    208.  —  Diagram    of 
THE  Pinal  Branches  of  the 
Mammalian  Bronchi. 
A,  Atrium;  B,  bronchus;  S, 
air-sac . — ( Miller.) 


THE  LARYNX 


337 


additional  branches  and  these  again  to  others,  and  the  mesen- 
chyme of  each  lobe  grows  in  between  the  various  branches.  At 
first  the  amount  of  mesenchyme  separating  the  branches  is  com- 
paratively great,  but  as  the  branches  continue  to  develop,  the 
growth  of  the  mesenchyme  fails  to  keep  pace  with  them,  so  that 
in  later  stages  the  terminal  enlargements  are  separated  from  one 
another  by  only  very  thin  partitions  of  mesenchyme,  in  which  the 
pulmonary  vessels  form  a  dense  network.  The  final  branching 
of  each  ultimate  bronchus  of 
bronchiole  results  in  the  forma- 
tion at  its  extremity  of  from  three 
to  five  enlargements,  the  atria 
(Fig.  209,  A)^  from  which  arise 
a  number  of  air-sacs  {S)  whose 
walls  are  pouched  out  into  sHght 
diverticula,  the  air-cells  or  alveoli. 
Such  a  combination  of  atria,  air- 
sacs,  and  air-cells  constitutes  a 
lobule,  and  each  lung  is  composed 
of  a  large  number  of  such  units. 

The  greater  part  of  the  original 
pulmonary  groove  becomes  con- 
verted into  the  trachea,  and  in 
the  mesenchyme  surrounding  it 
the  incomplete  cartilaginous  rings 
develop  at  about  the  eighth  or 
ninth  week.  The  cells  of  the  epithelial  lining  of  the  trachea  and 
bronchi  remain  columnar  or  cubical  in  form  and  become  ciliated 
at  about  the  fourth  month,  but  those  of  the  epitheHum  of  the  air- 
sacs  become  greatly  flattened  and  constitute  an  exceedingly  thin 
layer  of  pavement  epithelium. 

The  Development  of  the  Larynx.— The  opening  of  the  upper 
end  of  the  pulmonary  groove  into  the  pharynx  is  situated  at  first 
just  behind  the  fourth  branchial  furrow  and  is  surrounded  an- 
teriorly and  laterally  by  the  U-shaped  ridge  already  described 

(p.  296)  as  the  furcula,  this  separating  it  from  the  posterior  por- 
22 


Fig.  209. — Reconstruction  of 
THE  Opening  into  the  Larynx  in 
AN  Embryo  of  Twenty-eight  Days, 
Seen  from  Behind  and  Above,  the 
Dorsal  Wall  of  the  Pharynx  being 
Cut  Away. 

CO,  Cornicular,  and  cu,  cuneiform 
tubercle;  Ep,  epiglottis;  T,  unpaired 
portion  of  the  tongue. — (Kallius.) 


^^S  THE    LARYNX 

tion  of  the  tongue  (Fig.  i8o).  The  anterior  portion  of  this  ridge, 
which  is  apparently  derived  from  the  ventral  portions  of  the 
third  branchial  arch,  gradually  increases  in  height  and  forms  the 
epiglottis,  while  the  lateral  portions,  which  pass  posteriorly  into 
the  margins  of  the  pulmonary  groove,  form  the  ary epiglottic  folds. 
When  the  pulmonary  groove  separates  from  the  oesophagus,  the 
opening  of  the  trachea  into  the  pharynx  is  somewhat  slit-like  and 
is  bounded  laterally  by  the  aryepiglottic  folds,  whose  margins 
present  two  elevations  which  may  be  termed  the  cornicular  and 
cuneiform  tubercles  (Fig.  209,  co  and  cu,  and  Fig.  177).  The 
opening  is,  however,  for  a  time  almost  obHterated  by  a  thickening 
of  the  epithehum  covering  the  ridges,  and  it  is  not  until  the  tenth 
or  eleventh  week  of  development  that  it  is  re-established.    Later 


Pig.  210. — Reconstruction  of  the  Mesenchyme  Condensations  which  Repre- 
sent THE  Hyoid  and  Thyreoid  Cartilages  in  an  Embryo  of   Forty   Days. 
The  darkly  shaded  areas  represent  centers  of  chondrification.     c.ma.  Greater  cornu 
of  hyoid;  c.mi,  lesser  cornu;  Th,  thyreoid  cartilage. — (Kallius.) 

than  this,  at  the  middle  of  the  fourth  month  a  linear  depression 
makes  its  appearance  on  the  mesial  surface  of  each  aryepiglottic 
fold,  forming  the  beginning  of  the  ventricle,  and  although  at  first 
the  depression  lies  horizontally,  its  lateral  edge  later  bends  an- 
teriorly, so  that  its  surfaces  look  outward  and  inward.  The  lips 
which  bound  the  opening  of  the  ventricle  into  the  laryngeal 
cavity  give  rise  to  the  ventricular  and  vocal  folds. 

The  cartilages  of  the  larynx  can  be  distinguished  during  the 
seventh  week  as  condensations  of  mesenchyme  which  are  but 
indistinctly  separated  from  one  another.  The  thyreoid  cartilage  is 
represented  at  this  stage  by  two  lateral  plates  of  mesenchyme, 
separated  from  one  another  both  ventrally  and  dorsally,  and  each 


THE    LARYNX  339 

of  these  plates  undergoes  chondrifi cation  from  two  separate  centers 
(Fig.  210).  These,  as  they  increase  in  size,  unite  together  and 
send  prolongations  ventrally  which  meet  in  the  mid-ventral  line 
with  the  corresponding  prolongations  of  the  plates  of  the  opposite 
side,  so  as  to  enclose  an  area  of  mesench3nne  into  which  the 
chondrification  only  extends  at  a  later  period,  and  occasionally  fails 
to  so  extend,  producing  what  is  termed  a  foramen  thyreoideum. 

The  mesenchymal  condensations  which  represent  the  cricoid 
and  arytenoid  cartilages  are  continuous,  but  each  arytenoid  has  a 
distinct  center  of  chondrification,  while  the  cartilage  of  the  cricoid 
appears  as  a  single  ring  which  is  at  first  open  dorsally  and  only 
later  becomes  complete.  The  epiglottis  cartilage  resembles  the 
thyreoid  in  being  formed  by  the  fusion  of  two  originally  distinct 
cartilages,  from  each  of  which  a  portion  separates  to  form  the 
cuneiform  cartilages  (cartilages  of  Wrisherg)  which  produce  the  tu- 
bercles of  the  same  name  on  the  aryepiglottic  fold,  while  the  cor- 
niculate  cartilages  {cartilages  of  Santorini)  are  formed  by  the 
separation  of  a  small  portion  of  cartilage  from  each  arytenoid. 

The  formation  of  the  thyreoid  cartilage  by  the  fusion  of  two 
pairs  of  lateral  elements  finds  an  explanation  from  the  study  of  the 
comparative  anatomy  of  the  larynx.  In  the  lowest  group  of  the 
mammalia,  the  Montremata,  the  four  cartilages  do  not  fuse 
together  and  are  very  evidently  serially  homologous  with  the  car- 
tilages which  form  the  cornua  of  the  hyoid.  In  other  words,  the 
thyreoid  results  from  the  fusion  of  the  fourth  and  fifth  branchial 
cartilages.  The  cricoid,  in  its  development,  ptresents  such  striking 
similarities  to  the  cartilaginous  rings  of  the  trachea  that  it  is 
probably  to  be  regarded  as  the  uppermost  cartilage  of  that  series, 
but  the  epiglottis  seems  to  be  a  secondary  chondrification  in  the 
glossolaryngeal  fold  (Schaffer).  The  arytenoids  possibly  rep- 
resent an  additional  pair  of  branchial  cartilages,  such  as  occur 
in  the  lower  vertebrates  (Gegenbaur). 

These  last  arches  have  undergone  almost  complete  reduction  in 
the  mammalia,  the  cartilages  being  their  only  representatives,  but, 
in  addition  to  the  cartilages,  the  fourth  and  fifth  arches  have  also 
preserved  a  portion  of  their  musculature,  part  of  which  becomes 


340  LITERATURE 

transformed  into  the  muscles  of  the  larynx.  Since  the  nerve 
which  corresponds  to  these  arches  is  the  vagus,  the  supply  of  the 
larynx  is  derived  from  that  nerve,  the  superior  laryngeal  nerve 
probably  corresponding  to  the  fourth  arch,  while  the  inferior 
(recurrent)  answers  to  the  fifth. 

The  course  of  the  recurrent  nerve  finds  its  explanation  in  the 
relation  of  the  nerve  to  the  fourth  branchial  artery.  When  the  heart 
occupies  its  primary  position  ventral  to  the  floor  of  the  pharynx,  the 
inferior  laryngeal  nerve  passes  transversely  inward  to  the  larynx 
beneath  the  fourth  branchial  artery.  As  the  heart  recedes  the  nerve 
is  caught  by  the  vessel  and  is  carried  back  with  it,  the  portion  of  the 
vagus  between  it  and  the  superior  laryngeal  nerve  elongating  until  the 
origins  of  the  two  laryngeal  nerves  are  separated  by  the  entire  length 
of  the  neck.  Hence  it  is  that  the  right  recurrent  nerve  bends  upward 
behind  the  right  subclavian  artery,  while  the  left  curves  beneath  the 
arch  of  the  aorta  (see  Fig.  151), 

LITERATURE 

J.  M.  Flint:  "The  Development  of  the  Lungs,"  Amer.  Journ.  Anat.,  vi,  1906. 

J.  E.  Frazer:  "The  Development  of  the  Larynx,"  Journ.  Anat.  and  Phys.,  xliv, 

1910. 
E.  Goppert;  "Ueber  die  Herkunft  der  Wrisbergschen  Knorpels,"  MorphoL  Jahruch, 

XXI,  1894. 
W.  His:  "Zur  Bildungsgeschichte  des  Lungen  beim  menschiichen  Embryo,"  Archiv 

filr  Anat.  und  Physiol.,  Anat.  Abth.,  1887. 
E.  Kallius:  "Beitrage  zur  Entwickelungsgeschichte  des  Kehlkopfes,"  Anat.  Hefte, 

IX,  1897. 
K.  Kallius:  "Die  Entwickelung  des  menschiichen  Kehlkopfes,"  Verhandl.  der  Anat. 

Gesellsch.,  xn,  1898. 
A.  Lisser:  "Studies  on  the  Development  of  the  Human  Larynx,"  Amer.  Journ. 

Anat.,  XII,  1911.        / 
A.  Narath:  "Der  Bronchialbaum  der  Saugethiere  und  des  Menschen,"  Bibliotheca 

Medica,  Abth.  A,  Heft  3,  1901. 
J.  Schaffer:  "Zur  Histologic,  Histogenese  und  phylogenetischen  Bedeutung  der 

Epiglottis,"  Anat.  Hefte,  xxxiii,  1907. 
A.  SouLiE  AND  E.  Bardier:  "Recherches  sur  le  developpement  du  larynx  chez 

I'homme,"  Journ.  de  I' Anat.  et  de  la  Physiol.,  xliii,  1907. 


CHAPTER  XIII 

THE  DEVELOPMENT  OF  THE  URINOGENITAL  SYSTEM 

The  excretory  and  reproductive  systems  of  organs  are  so  closely 
related  in  their  development  that  they  must  be  considered  to- 
gether. They  both  owe  their  origin  to  the  mesoderm  which 
constitutes  the  intermediate  cell-mass  (p.  80),  this,  at  an  early 
period  of  development,  becoming  thickened  so  as  to  form  a  ridge 
projecting  into  the  dorsal  portion  of  the  coelom  and  forming  what 
is  known  as  the  Wolffian  ridge  (Fig.  211,  wr).  The  greater 
portion  of  the  substance  of  this  ridge  is  concerned  in-  the  de- 

^^^c 


\ 


J      t    "^c-; 


wr 


Fig.  211, — Transverse  Section  through  the  Abdominal  Region  of  a  Rabbit 

Embryo  of  12  mm. 

a,  Aorta;  gl,  glomerulus;  gr,  genital  ridge;  m,  mesentery;  nc,  notochord;  /,  tubule  of 

mesonephros;  wd.  Wolffian  duct;  wr.  Wolffian  ridge. — (Mihalkovicz.) 

velopment  of  the  primary  and  secondary  excretory  organs,  but 
on  its  mesial  surface  a  second  ridge  appears  which  is  destined  to 
give  rise  to  the  ovary  or  testis,  and  hence  is  termed  the  genital 
ridge  (gr). 

The  development  of  the  excretory  organs  is  remarkable  in  that 
three  sets  of  organs  appear  in  succession.  The  first  of  these,  the 
pronephros,  exists  only  in  a  rudimentary  condition  in  the  human 
embryo,  although  its  duct,  the  pronephric  or  Wolffian  duct,  under- 

341 


342  THE   PRONEPHROS 

goes  complete  development  and  plays  an  important  part  in  the 
development  of  the  succeeding  organs  of  excretion  and  also  in 
that  of  the  reproductive  organs.  The  second  set ,  the  mesonephros 
or  Wolffian  body,  reaches  a  considerable  development  during  em- 
bryonic life,  but  later,  on  the  development  of  the  final  set,  the 
definite  kidney  or  metanephros,  undergoes  degeneration,  portions 
only  persisting  as  rudimentary  structures  associated  for  the  most 
part  with  the  reproductive  organs. 

The  Development  of  the  Pronephros  and  the  Pronephric 
Duct. — The  first  portions  of  the  excretory  system  to  make  their 
appearance  are  the  pronephric  or  Wolffan  ducts,  which  develop  as 


im^  ,^f.  r/i. 

Fig.  212. — Transverse  Section  through  Chick  Embryo  of  about  Thirty-six 

Hours. 

en,  Endoderm;  im,  intermediate  cell  mass;  ms,  mesodermic  somite;  nc,  notochord;  so, 

somatic,  and  sp,  splanchnic  mesoderm;  wd.  Wolffian  duct. — (Waldeyer.) 

outgrowths  of  the  dorsal  walls  of  the  intermediate  cell  masses. 
At  first  the  outgrowths  are  solid  cords  of  cells  (Fig.  212,  wd),  but 
later  a  lumen  appears  in  the  center  of  each  and  the  canal  so  formed 
from  each  intermediate  cell  mass,  bending  backward  at  its  free 
end,  comes  into  contact  and  fuses  with  the  canal  from  the  next 
succeeding  segment.  Two  longitudinal  canals,  the  pronephric 
or  Wolffian  ducts,  are  thus  formed,  with  which  the  cavities  of  the 
intermediate  cell  masses  communicate.  The  formation  of  the 
ducts  begins  in  the  anterior  segments  before  the  segmentation  of 
the  posterior  portions  of  the  mesoderm  has  taken  place,  and  the 
further  backward  extension  of  the  ducts  takes  place  independently 
of  the  formation  of  excretory  tubules,  apparently  by  a  process  of 
terminal  growth.  The  free  end  of  each  duct  comes  into  intimate 
relation  with  the  ectoderm  above  it,  so  much  so  that  its  posterior 


THE   PRONEPHROS 


343 


portion  has  been  held  by  some  observers  to  be  formed  from  that 
layer,  but  it  seems  more  probable  that  the  relation  to  the  ectoderm- 
is  a  secondary  process  and  that  the  ducts  are  entirely  of  meso- 
dermal origin.  They  reach  the  cloaca  in  embryos  of  a  little  over 
4  mm.,  and  later  they  unite  with  that 
organ,  so  that  their  lumina  open  into 
its  cavity. 

The  pronephric  tubules  make  their 
appearance  in  embryos  of  about  1.7 
mm.,  while  as  yet  there  are  only  nine  or 
ten  mesodermic  somites,  and  they  are 
formed  from  the  intermediate  cell  masses 
of  the  seventh  to  the  thirteenth  or  four- 
teenth segment,  and  perhaps  from  those 
situated  still  more  anteriorly.  They 
attain  their  maximum  development  in 
embryos  of  about  3.5  mm.,  one  of  this 
size  having  an  arrangement  of  the  ex- 
cretory apparatus  as  shown  in  Fig.  213. 
On  the  left  side,  beginning  at  the  seventh 
segment  and  extending  back  to  the 
thirteenth,  are  a  number  of  pronephric 
tubules,  whose  inner  ends  have  united 
to  form  the  pronephric  or  Wolffian  duct. 
This,  however,  has  grown  backward  as 
far  as  the  sixteenth  segment,  into  the 
region  in  which  the  mesonephros  has 
begun  to  differentiate.  It  will  be  noted 
that  the  pronephric  tubules  are  not 
exactly  metameric  in  their  arrangement, 
some  of  the  segments  possessing  two. 
On  the  right  side  the  first  prone- 
phric tubule  occurs  in  the  ninth  segment,  but  in  the  two  preceding 
ones  the  degenerated  remains  of  two  additional  tubules  occur.  For 
the  pronephric  system  begins  to  degenerate  at  its  anterior  end 
almost  before  the  most  posterior  tubules  have  formed,  and  with 


Fig.  213. — Diagram  of 
THE  Arrangement  of  the 
Excretory  Apparatus  of  an 
Embryo  of  3.5  mm. 

The  unstippled  structures 
with  heavy  outlines  represent 
the  pronephric  system;  the 
solid  black  bodies,  degenerated 
pronephric  tubules;  the  stip- 
pled structures,  differentiating 
mesonephric  tubules,  termi- 
nating, in  the  unsegmented 
nephrogenic  c  o  r  d.  ^-  (7.  C. 
Watt.) 


344  THE    PRONEPHROS 

the  disappearance  of  the  tubules  there  is  a  disappearance  of  the 
corresponding  portions  of  the  Wolffian  duct.  In  the  embryo  from 
which  Fig.  213  was  drawn  the  excretory  apparatus  of  the  right  side 
was  a  little  further  advanced  in  development  than  that  of  the  left 
side,  and  hence  the  occurrence  of  degenerated  tubules  on  the 
former  only. 

Each  pronephric  tubule,  when  fully  formed,  consists  of  a 
portion  which  unites  it  to  the  Wolffian  duct,  and  opens  at  its 
other  end  into  an  enlargement,  the  pronephric  chamber  (Fig.  214, 
pc),  which,  on  its  part  opens  into  the  coelomic  cavity  by  means  of 
a  nephrostome  canal.     In  the  neighborhood  of  the  coelomic  open- 


FiG.  214. — Diagram  showing  the  Structure  of  a  Fully  Developed  Pronephric 

Tubule. 
Ao,  Aorta;  Coe,  coelom;  ec.  Ectoderm;  eg,  external  glomerulus;  en,  endoderm;  Ms, 
mesodermic  somite;  N,  nervous  system;  n,  nephrostome;  nc,  notochord;  pc,  prone- 
phric chamber;  Wd,  Wolffian  duct. — {Modified  from  Felix.) 

ing,  or  nephrostome,  an  outgrowth  of  the  coelomic  epithelium  is 
formed,  and  a  branch  from  the  aorta  penetrates  into  this  to  form  a 
stalked  external  glomerulus  lying  free  in  the  coelomic  cavity  (Fig. 
214,  eg).  Internal  glomeruh,  such  as  occur  in  connection  with  the 
mesonephric  tubules  do  not  occur  in  the  pronephros  of  the  human 
embryo,  and  this  fact,  together  with  the  presence  of  external 
glomeruli  and  the  participation  of  the  tubules  in  the  formation  of 
the  Wolffian  duct,  serve  to  distinguish  the  pronephros  from  the 
meson  ephros. 

The  pronephric  tubules  are,  as  has  been  stated,  transitory  struc- 
tures and  by  the  time  the  embryo  has  reached  a  length  of  about 
5  mm.  they  have  all  disappeared.     Before  their  disappearance 


THE    MESONEPHROS 


345 


Wd 


is  complete,  however,  a  second  series  of  tubules  has  commenced 
to  develop,  forming  what  is  termed  the  mesonephros  or  Wolffian^ 
body. 

The  Development  of  the  Mesonephros. — The  pronephric 
duct  does  not  entirely  disappear  with  the  degeneration  of  the 
pronephric  tubules,  but  persists  to  serve  as  the  duct  for  the  meso- 
nephros and  to  play  an  important  part  in  the  development  of 
the  metanephros  also.  In  the  Wolffian  ridge  there  appear  in 
embryos  of  between  3  and  4  mm.  a  number  of  coiled  tubules,  which 
arise  by  some  of  the  cells  of  the 
ridge  aggregating  to  form  solid 
cords,  at  first  entirely  uncon- 
nected with  either  the  coelomic 
epithelium  or  the  Wolffian  duct. 
Later  the  cords  become  con- 
nected with  the  coelomic  epithe- 
lium and  acquire  a  lumen,  and 
near  the  coelomic  end  of  the 
tubule,  at  a  region  correspond- 
ing to  the  chamber  of  a  prone- 
phric tubule,  a  condensation  of 
the  mesenchyme  of  the  Wol- 
ffian ridge  occurs  to  form  a  glom- 
erulus into  which  a  branch  ex- 
tends from  the  neighboring  aorta.  The  tubules  finally  acquire 
connection  with  the  Wolffian  duct  and  at  the  same  time  lose  their 
connections  with  the  coelomic  epithelium,  their  nephros tomes 
being  accordingly  but  transitory  structures.  The  tubules  rapidly 
increase  in  length  and  become  coiled,  and  the  glomeruli  project 
into  their  cavities,  pushing  in  front  of  them  the  wall  of  the  tubule 
so  that  it  has  the  appearance  represented  in  Fig.  215. 

In  its  anterior  portion  the  Wolffian  ridge  is  formed  from  distinct 
intermdiate  cell  masses,  but  posterior  to  the  tenth  segment  it 
becomes  distinguishable  from  the  rest  of  the  mesoderm  before  this 
has  become  segmented,  and,  failing  to  undergo  transverse  division 
into  segments,  it  forms  a  continuous  column  of  cells,  known  as  the 


Fig.  215. — Transverse  Section  of 
THE  Wolffian  Ridge  of  a  Chick  Em- 
bryo OF  Three  Days. 

ao,  Aorta;  gl,  glomerulus;  gr,  genital 
ridge;  mes,  mesentery;  tnt,  mesonephric 
tubule;  vc,  cardinal  vein;  Wd,  Wolffian 
duct. — (Mihalkoricz.) 


346 


THE   MESONEPHROS 


nephrogenic  cord.  The  anterior  tubules  of  the  mesonephros  make 
their  appearance  in  the  intermediate  cell  masses  belonging  to  the 
sixth  cervical  segment,  its  tubules  thus  overlapping  those  of  the 
pronephros,  and  from  this  level  they  appear  in  all  succeeding  seg- 
ments and  in  the  nephrogenic  cord  as  far  back  as  the  region  of  the 
third  or  fourth  lumbar  segment,  where  the  cord  is  partially  inter- 
rupted. This  interruption  marks  the  dividing  line  between  the 
mesonephric  and  metanephric  portions  of  the  cord,  the  portions 
posterior  to  it  being  destined  to  give  rise  to  the  metanephros. 


Fig.  216. — Urinogenital  Apparatus  of  a  Male  Pig  Embryo  of  6  cm. 

ao.  Aorta;  h,  bladder;  gh,  gubernaculum  testis;  fe,  kidney;  md,  Mullerian  duct;  sr, 

suprarenal  body;  t,  testis;  w,  "Wolffian  body;  wd,  Wolffian  duct. — (Mihalkovicz.) 

But,  as  is  the  case  with  the  pronephros,  the  entire  series  of  meso- 
nephric tubules  is  never  in  existence  at  any  one  time,  a  degenera- 
tion of  the  anterior  ones  supervening  even  before  the  posterior 
ones  have  differentiated,  and  the  degeneration  proceeds  to  such  an 
extent  that  in  an  embryo  of  about  21  mm.  all  the  tubules  of  the 
cervical  and  thoracic  segments  have  disappeared,  only  those  of 
the  lumbar  segments  persisting. 


THE    METANEPHROS  347 

This  does  not  mean,  however,  that  the  number  of  persisting 
tubules  corresponds  with  that  of  the  segments  in  which  they  occur ,- 
for  the  tubules  are  not  segmental  in  their  arrangement,  but  are 
much  more  numerous  than  such  an  arrangement  would  allow. 
Two,  three,  or  even  as  many  as  nine  may  correspond  with  the 
extent  of  a  mesodermic  somite  and  when  the  reduction  is  complete 
in  an  embryo  of  21  mm.,  where  only  the  tubules  corresponding 
with  four  or  five  segments  remain,  they  may  number  twenty-six 
in  each  mesonephros  (Felix).  This  arrangement  of  the  tubules 
together  with  the  size  which  they  assume  when  fully  developed 
brings  it  about  that  the  Wolffian  ridges  become  somewhat  volu- 
minous structures  in  their  mesonephric  portions,  projecting  mark- 
edly into  the  coelomic  cavity  (Fig.  216).  Each  is  attached  to  the 
dorsal  wall  of  the  body  by  a  distinct  mesentery  and  has  in  its 
lateral  portion,  embedded  in  its  substance,  the  Wolffian  duct, 
while  on  its  mesial  surface  anteriorly  is  the  but  slightly  developed 
genital  ridge  (t) .  This  condition  is  reached  in  the  human  embryo 
at  about  the  sixth  or  seventh  week  of  development,  and  after  that 
period  the  mesonephros  again  begins  to  undergo  rapid  degenera- 
tion, so  that  at  about  the  sixteenth  week  nothing  remains  of  it 
except  the  duct  and  a  few  small  rudiments  whose  history  will  be 
given  later. 

The  Development  of  the  Metanephros. — The  first  indication 
of  the  metanephros  or  permanent  kidney  is  a  tubular  outgrowth 
from  the  dorsal  surface  of  the  Wolffian  duct  shortly  before  its 
entrance  into  the  cloaca  (Fig.  172).  When  first  formed  this  out- 
growth lies  lateral  to  the  posterior  portion  of  the  Wolffian  ridge, 
which,  as  has  already  been  noted  (p.  346),  is  separated  from  the 
portion  that  gives  to  the  mesonephros.  This  terminal  portion  of 
the  ridge  forms  what  is  termed  the  metanephric  blastema  and  in 
embryos  of  7  mm.  it  has  come  into  relation  with  the  outgrowth 
from  the  Wolffian  duct  and  covers  its  free  extremity  as  a  cap. 
Since  both  the  blastema  and  the  outgrowth  from  the  Wolffian  duct 
take  part  in  the  formation  of  the  uriniferous  tubules,  these  have  a 
double  origin. 

The  outgrowth  from  the  Wolffian  duct  as  it  continues   to 


348 


THE  METANEPHROS 


elongate  comes  to  lie  dorsal  to  the  mesonephros,  carrying  the 
cap  of  blastema  with  it,  and  it  soon  assumes  a  somewhat  club- 
shaped  form,  its  terminal  enlargement  or  ampulla  forming  what 
may  be  termed  the  primary  renal  pelvis,  while  the  remainder 
represents  the  ureter.  The  primary  renal  pelvis  then  becomes  bent 
laterally  so  that  is  axis  lies  at  an  angle  with  that  of  the  ureter  and  it 
becomes  distinctly  bilobed  (Fig.  217,  A)  each  lobe  having  a  cap 
of  blastema,  the  original  metanephric  blastema  having  divided 
into  two  portions.     From  each  lobe  there  are  then  pushed  out  from 


Fig.  217. — Three  reconstructions  showing  the  development  of  the  secondary 
collecting  tubules  as  branches  from  the  distal  end  of  the  ureter  in  the  human  em- 
bryo.    The  caps  of  metanephric  blastema  are  not  represented. 

three  to  six,  usually  four,  outgrowths,  (Fig.  217,5)  which  represent 
primary  collecting  tubules,  and  on  their  formation  the  two  caps  of 
metanephric  blastema  undergo  divisions  into  as  many  parts  as 
there  are  outgrowths  from  the  lobes,  each  outgrowth  thus  having 
its  own  cap  of  blastema.  As  soon  as  each  primary  tubule  has 
reached  a  certain  length  its  free  extremity  begins  to  bud  off  from 
two  to  four  secondary  collecting  tubules,  (Fig.  217,  C)  and  a  further 
corresponding  division  of  the  metanephric  blastema  takes  place. 
In  their  turn  these 'secondary  tubules  similarly  bud  out  tertiary 
collecting  tubules,  their  development  being  accompanied  by  another 


THE    METANEPHROS 


349 


fragmentation  of  the  blastema  and  so  the  process  goes  on  until 
about  the  fifth  fetal  month,  the  number  of  generations  of  collect-.^ 
ing  tubules  formed  being  between  eleven  and  thirteen,  each  tubule 
of  the  final  generation  having  its  cap  of  blastema. 

In  this  way  there  is  formed  a  complicated  branching  system 
of  tubules,  all  of  which  ultimately  communicate  with  the  primary 
renal  pelvis  and  all  of  which  have,  in  the  last  analysis,  had  their 


Fig,  2 1 8, — Four  Stages  of  Development  of  a  Uriniferous  Tubule  of  a  Cat. 
A,  Arched  collecting  tubule,  C,  distal  convoluted  tubule;  C,  proximal  convoluted 
tubule;  H,  loop  of  Henle;  M,  glomerulus;  T,  renal  vesicle;  V,  ampulla  (drawn  from 
reconstructions  prepared  by  G.  C.  Huber). 

origin  from  the  Wolffian  duct.  They  represent,  however,  only 
the  collecting  portions  of  the  uriniferous  tubules,  their  excreting 
portions  having  yet  to  form,  and  these  take  their  origin  from  the 
metanephric  blastema. 

When  the  terminal  collecting  tubules  have  been  formed  the 
blastemic  cap  in  connection  with  each  one  condenses  to  form  a 
renal  vesicle  (Fig.  21S,  A,  T),  which  is  at  first  solid,  but  later  be- 
comes hollow  and  proceeds  to  elongate  to  an  S-shaped  tubuk, 
one  end  of  which  becomes  continuous  with  the  neighboring  am- 
pulla (Fig.  218,  B),  and  in  the  space  enclosed  by  what/ may  be 


350  THE    METANEPHROS 

termed  the  lower  loop  of  the  S  a  collection  of  mesenchyme  cells 
appears,  into  which  branches  penetrate  at  an  early  stage  from  the 
renal  artery  to  form  a  glomerulus,  the  neighboring  walls  of  the 
tubule  becoming  exceedingly  thin  and  being  transformed  into  a 
capsule  of  Bowman.  The  upper  loop  of  the  S  now  begins  to 
elongate  (Fig.  218,  C),  growing  toward  the  hilus  of  the  kidney, 
parallel  to  the  branch  of  the  outgrowth  from  the  Wolffian  duct  to 
which  it  is  attached  and  between  this  and  the  glomerulus,  and 
forms  a  loop  of  Henle.  From  the  portion  of  the  horizontal  limb 
of  the  S  which  lies  between  the  glomerulus  and  the  descending  limb 
of  the  loop  of  Henle  the  proximal  convoluted  tubule  (C)  arises, 
while  the  distal  convoluted  and  the  arched  collecting  tubules 
(C  and  A)  are  formed  from  the  uppermost  portion  of  the  upper 
loop  (Fig.  218,  D).  The  entire  length  of  each  uriniferous  tubule 
from  Bowman's  capsule  to  the  arched  collecting  tubule  inclusive 
is  thus  derived  from  a  renal  vesicle,  that  is  to  say,  from  the 
metanephric  blastema. 

Since  the  tubules  of  the  kidney  are  formed  by  the  union  of  two 
originally  distinct  structures  it  is  conceivable  that[in  the  cases  of  certain 
tubules  there  may  be  a  failure  of  the  union.  The  blastemic  portions 
of  the  tubules  would,  nevertheless,  continue  their  development  and 
become  functional  and,  since  there  would  be  no  means  of  escape  for 
the  secretion,  the  result  would  be  a  cystic  kidney.  Occasionally  the 
two  blastemata  of  opposite  sides  fuse  across  the  middle  line,  the  re- 
sult being  the  formation  of  a  single  transverse  or  horse-shoe  shaped 
kidney  or,  what  is  much  rarer,  the  blastema  of  one  side  may  cross 
the  middle  line  to  fuse  with  that  of  the  other,  the  result  being  an 
apparently  single  kidney  with  two  ureters  which  open  normally  into 
the  bladder. 

The  primary  renal  pelvis  is  the  first  formed  ampulla  and  does 
not  exactly  represent  the  definitive  pelvis.  This  is  produced 
partly  by  the  enlargement  of  the  primary  pelvis  and  greatly  by  the 
enlargement  of  the  collecting  tubules  of  the  first  four  generations, 
those  of  the  third  and  fourth  generations  later  being  taken  up  or 
absorbed  into  those  of  the  second  generation,  so  that  the  tubules 
of  the  fifth  generation  appear  to  open  directly  into  those  of  the 
second,  which  form  the  calices  minores,  while  those  of  the  first 


THE   MULLERIAN   DUCT  35 1 

constitute  the  calices  majores.  In  some  kidneys  the  process  of 
reduction  of  the  earlier  formed  collecting  tubules  proceeds  a  step 
further,  those  of  the  first  generation  being  taken  up  into  the 
primary  renal  pelvis,  the  secondaries  then  forming  a  series  of  short 
calices  arising  from  a  single  pelvic  cavity. 

At  about  the  tenth  week  of  development  the  surface  of  the 
human  kidney  becomes  marked  by  shallow  depressions  into  lobes, 
of  which  there  are  about  eighteen,  one  corresponding  to  each  of 
the  groups  of  tubules  which  arise  from  the  same  renal  vesicle. 
This  lobation  persists  until  after  birth  and  then  disappears  com- 
pletely, the  surface  of  the  kidney  becoming  smooth. 

The  Development  of  the  Miillerian  Duct  and  of  the  Genital 
Ridge. — At  the  time  when  the  Wolffian  body  has  almost  reached 
its  greatest  development  the  Wolffian  ridge  is  distinctly  divided 
into  three  portions  (Fig.  219),  a  median  or  mesonephric  portion 
attached  to  the  body  wall,  a  lateral  or  tubal  portion  containing 
the  Wolffian  duct  and  attached  to  the  mesonephric  portion,  and 
a  genital  portion,  formed  by  the  genital  ridge  and  also  attached  to 
the  mesonephric  portion,  but  to  its  medial  surface.  In  the  tubal 
portion  a  second  longitudinal  duct,  known  as  the  Miillerian  duct 
(Fig.  219,  Md),  makes  its  appearance.  Near  the  anterior  end  of 
each  Wolffian  ridge  there  is  formed  on  the  free  edge  of  the  tubal 
portion  an  invagination  of  the  peritoneal  covering,  and  by  the 
proliferation  of  the  cells  at  its  tip  this  invagination  gradually 
extends  backward  in  the  substance  of  the  tubal  portion  and  reaches 
the  cloaca  in  embryos  of  about  22  mm.  The  primary  peritoneal 
invagination  becomes  the  abdominal  ostium  of  the  Miillerian 
duct,  the  backward  prolongation  forming  the  duct  itself. 

In  Fig.  219  it  will  be  seen  that  the  tubal  portion  of  the  left 
Wolffian  ridge  is  somewhat  bent  inward  toward  the  median  line 
and  in  the  lower  parts  of  their  extent  this  becomes  more  pro- 
nounced in  both  tubal  portions  until  finally  their  free  edges  come 
in  contact  and  fuse  in  the  median  fine,  while  at  the  same  time  their 
lower  edges  fuse  with  the  floor  of  the  coelomic  cavity.  In  this 
way  a  transverse  partition  is  formed  across  what  will  eventually 
be  the  pelvis  of  the  adult,  this  cavity  being  thus  divided  into  two 


352 


THE    GENITAL    RIDGE 


compartments,  a  posterior  one  containing  the  lower  portion  of 
the  intestine  and  an  anterior  one  containing  the  bladder.     With 


<^^ 


r-' 


M 


N 


-^ 


^i     ^ 


M 


Fig.  219. — Transverse  Section  through  the  Abdominal  Region  of  an  Embryo 

OF    25    MM. 

Ao,  Aorta;  B,  bladder;  7,  intestine;  L,  liver;  M,  muscle;  Md,  Miillerian  duct;  Isl , 
spinal  cord;  Ov,  ovary;  KA,  rectus  abdominis;  Sg,  spinal  ganglion;  UA,  umbilical 
artery;  Ur,  ureter;  F,  vertebra;  W,  Wolflfian  body;  WD,  Wolffian  duct. — (Keibel.) 

the  formation  of  this  transverse  fold,  which  is  represented  by  the 
broad  ligament  in  the  female,  the  Miillerian  ducts  of  opposite  sides 


THE    GENITAL   RIDGE  .  353 

are  brought  into  contact  and  finally  fuse  in  the  lower  portions  of 
their  course  to  form  an  unpaired  utero-vaginal  canal. 

Upon  the  lateral  surface  of  the  mesonephric  portion  of  the 
Wolffian  ridge  a  longitudinal  elevation  is  formed  at  about  this 
time.  It  is  the  inguinal  fold  and  on  the  union  of  the  transverse 
fold  with  the  floor  of  the  coelomic  cavity  it  comes  into  contact 
and  fuses  with  the  lower  part  of  the  anterior  abdominal  wall, 
just  lateral  to  the  lateral  border  of  the  rectus  abdominis  muscle. 
In  the  substance  of  the  fold  the  mesenchyme  condenses  to  form 
a  ligament-Hke  cord,  the  inguinal  ligament,  whose  further  history 
will  be  considered  later  on. 

The  genital  ridge  makes  its  appearance  as  a  band-Hke  thicken- 
ing of  the  epithelium  covering  the  mesial  surface  of  the  Wolffian 
ridge  (Fig.  211,  gr).  Later  columns  of  cells  grow  down  from  the 
thickening  into  the  substance  of  the  Wolffian  ridge,  displacing 
the  mesonephric  tubules  to  a  greater  or  less  extent.  These 
columns  are  composed  of  two  kinds  of  cells:  (i)  smaller  epithelial 
cells  with  a  relatively  small  amount  of  cytoplasm  and  (2)  large, 
spherical  cells  with  more  abundant  and  clear  cytoplasm  known  as 
sex-cells.  The  growth  of  the  cell-columns  down  into  the  substance 
of  the  Wolffian  body  does  not  take  place,  however,  to  an  equal 
extent  in  all  portions  of  the  length  of  the  genital  ridge.  Indeed, 
three  regions  may  be  recognized  in  the  ridge;  an  anterior  one  in 
which  a  relatively  small  number  of  cell-columns,  extending  deeply 
into  the  stroma,  is  formed;  a  middle  one  in  which  numerous 
columns  are  formed;  and  a  posterior  one  in  which  practically 
none  are  formed.  The  first  region  has  been  termed  the  rete  region 
and  its  cell-columns  the  rete-cords,  the  second  region  the  sex-gland 
region  and  its  columns  the  sex-cords,  and  the  posterior  region  is 
the  mesenteric  region  and  plays  no  part  in  the  actual  formation  of 
the  ovary  or  testis. 

It  has  been  found  that  in  the  lower  vertebrates  and  also  in 
mammals  (Allen,  Rubaschkin)  the  sex-cells  make  their  appear- 
ance, not  in  the  epithelium  of  the  genital  ridge,  but  in  the  endo- 
derm  of  the  digestive  tract.  Thence  they  wander  into  the 
mesentery  and  eventually  into  the  peritoneum  covering  the  mesial 

23 


354 


THE   TESTIS 


surface  of  the  Wolffian  ridge  and  thus  into  the  epithelium  of  the 
genital  ridge.  Fuss  has  recently  obtained  evidence  that  the  sex- 
cells  of  the  human  embryo  have  a  similar  origin  and  undergo  a 
similar  migration. 

The  various  steps  in  the  differentiation  of  the  reproductive 
organs  so  far  described  occur  in  all  embryos,  no  matter  what  their 
future  sex  may  be.  The  later  stages,  however,  differ  according 
to  sex,  and  consequently  it  will  be  necessary  to  follow  the  further 


Fig.  220. — Section  through  the  Testis  and  the  Broad  Ligament  of  the  Testis 

OF  an  Embryo  of  5.5  mm. 
ep.  Epithelium;  md,  Mullerian  duct;  mo,  mesorchium;  re,  rete-cords;  sc,  sex-cords; 
wd.  Wolffian  duct. — (Mihalkovicz.) 

development  first  of  the  testis  and  then  of  the  ovary,  the  changes 
that  take  place  in  the  ducts  and  other  accessory  structures  being 
reserved  for  a  special  section. 

The  Development  of  the  Testis. — At  about  the  fourth  or  fifth 
week  there  appears  in  the  sex-gland  of  the  genital  ridge  a  struc- 
ture which  serves  to  characterize  the  region  as  a  testis.  This  is  a 
layer  of  somewhat  dense  connective  tissue  which  grows  in  between 
the  epitheHal  and  stroma  layers  of  the  sex-gland  region  and  gradu- 
ally extends  around  almost  the  entire  sex-gland  to  form  the 
tunica  albuginea.  By  its  development  the  sex-cords  are  separated 
from   the  epithelium,  which  later  becomes  much  flattened  and 


THE   TESTIS 


355 


eventually  almost  disappears.     Shortly  after  the  appearance  of 
the  albuginea  the  sex-cords  unite"'to"'form  a  complicated  network 
and  the  rete-cords  grow  backward  along  the  line  of  attachment  of- 
the  testis  to  the  mesonephric  portion  of  the  Wolffian  ridge,  coming 
to  lie  in  the  hilus  of  the  testis  (Fig.  220).     They  then  develop  a 


Mc     — 


ep 


—  ^ 


Mn 


PJ 


5  I 


I 


Pig.  221. — ^Longitudinal  Section  of  the  Ovary  of  an  Embryo  Cat  of  9.4  cm. 
cor.  Cortical  layer;  ep,  epoophoron;  Mc,  medullary  cords;  Mn,  mesonephros ;  pf, 
peritoneal  fold  containing  Fallopian  tube;  R,  rete;  T,  Fallopian    tube. — (Coert,  from 
Buhler.) 

lumen  and  send  ofif  branches  which  connect  with  the  sex-cord 
reticulum  and  they  also  make  connection  with  the  glomerular 
portions  of  the  tubules  belonging  to  the  anterior  part  of  the 
mesonephros.  Since  like  the  sex-cords,  they  have  by  this  time 
separated  from  the  epithehum  that  gave  rise  to  them,  they  now 
extend  between  the  sex- cord  reticulum  and  the  anterior  mesoneph- 


356  THE    OVARY 

ric  tubules.  Certain  portions  of  the  sex-cords  now  begin  to  break 
down  leaving  other  portions  to  form  convoluted  stems  which 
eventually  become  the  seminiferous  tubules,  while  from  the  rete- 
cords  are  formed  the  tubuli  recti  and  rete  testis,  by  which  the 
spermatozoa  are  transmitted  to  the  mesonephric  tubules  and 
so  to  the  Wolffian  duct  (see  p.  358). 

The  development  of  the  seminiferous  tubules  is  not,  however, 
completed  until  puberty.  The  stems  derived  from  the  sex-cords 
form  cylindrical  cords,  between  which  He  stroma  cells  and  in- 
terstitial cells  derived  from  the  stroma;  but  until  puberty  these 
cords  remain  solid,  a  lumen  developing  only  at  that  period.  The 
cords  contain  the  same  forms  of  cells  as  were  described  as  occurring 
in  the  epithelium  of  the  germinal  ridge,  and  while  in  the  early 
stages  transitional  forms  seem  to  occur,  in  later  periods  the  two 
varieties  of  cells  are  quite  distinct,  the  sex-cells  becoming  sperma- 
togonia (see  p.  14)  and  being  the  mother  cells  of  the  spermatozoa, 
while  the  remaining  epithelial  cells  perhaps  become  transformed 
into  the  connective-tissue  walls  of  the  tubules. 

^  The  Development  of  the  Ovary. — In  the  case  of  the  ovary,  after 
the  formation  of  the  sex-cords,  connective  tissue  grows  in  between 
these  and  the  epithelium,  forming  a  layer  equivalent  to  the  tunica 
albuginea  of  the  testis.  It  is,  however,  a  much  looser  tissue  than 
its  homologue  in  the  male,  and,  indeed,  does  not  completely  isolate 
the  sex-cords  from  the  epithelium,  although  the  majority  of  the 
cords  are  separated  and  sink  into  the  deeper  portions  of  the  ovary 
where  they  form  what  have  been  termed  the  medullary  cords.  In 
the  meantime  the  germinal  epithelium  has  continued  to  bud  off 
cords  which  unite  to  form  a  cortical  layer  of  cells  lying  below  the 
epithelium  and  separated  from  the  medullary  cords  by  the  tunica 
albuginea  (Fig.  221). 

Later  the  cortical  layer  becomes  broken  up  by  the  ingrowth  of 
stroma  tissue  into  spherical  or  cord-like  masses,  consisting  of  sex- 
cells  and  epithelial  cells  (Fig.  222).  The  invasion  of  the  stroma 
continuing,  these  spheres  or  cords  {Pfiuger^s  cords)  become  divided 
into  smaller  masses,  the  primary  ovarian  follicles,  each  of  which 
consists  as  a  rule  of  a  single  sex-cell  surrounded  by  a  number  of 


THE    OVARY 


357 


epithelial  cells,  the  whole  being  enclosed  by  a  zone  of  condensed 
stroma  tissue,  which  eventually  becomes  richly  vascularized  and 
forms  a  theca  folliculi  (Fig;  lo).  The  epithelial  cells  in  each^ 
follicle  are  at  first  comparatively  few  in  number  and  closely 
surround  the  sex-cell  (Fig.  222,  f),  which  is  destined  to  become 
an  ovum,  but  in  certain  of  the  follicles  they  undergo  an  increase  by 
mitosis,  becoming  extremely  numerous,  and  later  secrete  a  fluid, 
the  liquor  folliculi,  which  collects  at  one  side  of  the  follicle  and 
eventually  forms  a  considerable 
portion  of  its  contents.  The 
follicular  cells  are  differentiated 
by  its  appearance  into  the  stratum 
granulosum,  which  surrounds  the 
wall  of  the  follicle,  and  the  discus 
proligerus,  in  which  the  ovum  is 
embedded  (Fig.  10,  dp),  and  the 
cells  which  immediately  surround 
the  ovum,  becoming  cylindrical 
in  shape,  give  rise  to  the  corona 
radiata  (Fig.  11,  cr). 

A  somewhat  similar  fate  is 
shared  by  the  medullary  cords, 
these  also  breaking  up  into  a  num- 
ber of  follicles,  but  sooner  or  later 
these  follicles  undergo  degeneration  so  that  shortly  after  birth 
practically  no  traces  of  the  cords  remain.  It  must  be  noted 
that  degeneration  of  the  follicles  formed  from  the  cortical  layer 
also  takes  place  even  during  fetal  life  and  continues  to  occur 
throughout  the  entire  periods  of  growth  and  functional  activity, 
numerous  atretic  follicles  being  found  in  the  ovary  at  all  times. 
Indeed  it  would  seem  that  degeneration  is  the  fate  of  the  great 
majority  of  the  folHcles  and  sex-cells  of  the  ovary,  but  few  ova 
coming  to  maturity  during  the  life-time  of  any  individual. 

Rete  cords  developed  from  the  rete  portion  of  the  germinal 
ridge  occur  in  connection  with  the  ovary  as  well  as  with  the  testis 
and  form  a  rete  ovarii  (Fig.  221,  R).     They  do  not,  however,  extend 


Fig.  222. — Section  of  the  Ovary  of 
A  Ne^-born  Child. 
a,  Ovarial  epithelium;  b,  proximal 
part  of  a  Pfliiger's  cord;  c,  sex-cell  in 
epithelium;  d  and  e,  spherical  masses;/, 
primary  follicle;  g,  blood-vessel. — (From 
Gegenbaur,  after  Waldeyer.) 


358  THE    GENITAL   DUCTS 

SO  deeply  into  the  ovary,  remaining  in  the  neighborhood  of  the 
mesovarium,  and  they  do  not  become  tubular,  but  resemble  closely 
the  medullary  cords  with  which  they  are  serially  homologous. 
They  separate  from  the  epithelium  and  make  connections  with  the 
glomeruli  of  the  anterior  portion  of  the  mesonephros,  on  the 
one  hand,  and  on  the  other  with  medullary  cords,  and  in  later 
stages  show  a  tendency  to  break  up  into  primary  follicles,  which 
early  degenerate  and  disappear  like  those  of  the  medullary  cords. 

The  Transformation  of  the  Mesonephros  and  the  Ducts. — 
At  one  period  of  development  there  are  present,  as  representative 
of  the  urinogenital  apparatus,  the  Wolffian  body  (mesonephros) 
and  duct,  the  Mullerian  duct,  and  the  developing  ovary  or  testis. 
Such  a  condition  forms  an  indifferent  stage  from  which  the  de- 
velopment proceeds  in  one  of  two  directions  according  as  the 
genital  ridge  becomes  a  testis  or  an  ovary,  the  Wolffian  body  in 
part  undergoing  degeneration  and  in  part  persisting  to  form  organs 
which  for  the  most  part  are  rudimentary,  while  in  the  female  the 
Wolffian  duct  also  degenerates  except  for  certain  rudiments  and 
in  the  male  the  Mullerian  duct  behaves  similarly. 

In  the  Male. — ^It  has  been  seen  that  the  Wolffian  body,  through 
the  rete  cords,  enters  into  very  intimate  relations  with  the  testis , 
and  it  may  be  regarded  as  divided  into  two  portions,  an  upper 
genital  and  a  lower  excretory.  In  the  male  the  genital  portion 
persists  in  its  entirety,  serving  as  the  efferent  ducts  of  the  testis, 
which,  beginning  in  the  spaces  of  the  rete  testis,  already  shown  to 
be  connected  with  the  capsules  of  Bowman,  open  into  the  upper 
part  of  the  Wolffian  duct  and  form  the  globus  major  of  the  epidid- 
ymis. The  excretory  portion  undergoes  extensive  degeneration,  a 
portion  of  it  persisting  as  a  mass  of  coiled  tubules  ending  bUndly 
at  both  ends,  situated  near  the  head  of  the  epidid3rmis  and  known 
as  the  paradidymis  or  organ  of  Giraldes,  while  a  single  elongated 
tubule,  arising  from  the  portion  of  the  Wolffian  duct  which  forms 
the  globus  minor  of  the  epididymis,  represents  another  portion  of 
it  and  is  known  as  the  vas  aberrans. 

The  Wolffian  duct  is  retained  complete,  the  portion  of  it  nearest 
the  testis  becoming  greatly  elongated  and  thrown  into  numerous 


THE    GENITAL   DUCTS  359 

coils,  forming  the  body  and  globus  minor  of  the  epididymis,  while 
the  remainder  of  it  is  converted  into  the  vas  deferens  and  the  diictus 
ejaculatorius.  A  lateral  outpouching  of  the  wall  of  the  duct  to 
form  a  longitudinal  fold  appears  at  about  the  thirteenth  week  and 
gives  rise  to  the  vesicula  seminalis,  the  lateral  position  of  the  out- 
growth explaining  the  adult  position  of  the  vesiculae  lateral  to  the 
vasa  deferentia.  At  about  the  fourteenth  week  evaginations  ap- 
pear in  the  wall  of  the  outgrowth  and  somewhat  later  the  lower 
portion  of  each  Wolffian  duct  enlarges  to  form  the  ampulla,  the 
adult  structure  of  the  vesicular  apparatus  being  acquired  at  about 
the  twenty-fifth  week  (embryos  of  220  mm.  vertex-breech  length). 

With  the  MUllerian  ducts  the  case  is  very  different,  since  they 
disappear  completely  throughout  the  greater  part  of  their  course 
only  their  upper  and  lower  ends  persisting,  the  former  giving  rise 
to  a  small  sac-like  body,  the  sessile  hydatid  of  Morgagni,  attached 
to  the  upper  end  of  each  testis  near  the  epididymis.  It  has  been 
seen  (p.  351)  that  the  lower  ends  of  the  MUllerian  ducts,  in  the 
male  as  well  as  the  female,  fuse  to  form  the  utero- vaginal  canal, 
and  the  lower  portion  of  this  also  persists  to  form  what  is  termed 
the  uterus  masculinus,  although  it  corresponds  to  the  vagina  of  the 
female  rather  than  to  the  uterus.  It  is  a  short  cylindrical  pouch 
of  varying  length,  that  opens  into  the  urethra  at  the  bottom  of  a 
depression  known  as  the  utriculus  prostaticus  {sinus  pocularis). 

The  transverse  pelvic  partition,  produced  by  the  union  of  the 
two  tubal  portions  of  the  Wolffian  body,  is  formed  in  the  male 
embryo,  but  at  an  early  stage  its  anterior  surface  fuses  with  the 
posterior  surface  of  the  bladder  and  consequently  there  is  in  the 
male  no  pelvic  compartment  equivalent  to  the  vesico-uterine  pouch 
of  the  female.  The  male  recto-vesical  pouch  is,  however,  the 
homologue  of  the  recto-uterine  pouch  of  the  female. 

The  formation  of  the  inguinal  ligament  on  the  surface  of  the 
mesonephros  has  been  described  on  p.  353.  On  the  degeneration 
of  the  mesonephros  the  layer  of  peritoneum  that  covered  it  per- 
sists to  form  a  mesorchium  extending  from  the  body  wall  to  the 
hilus  of  the  testis  and  the  inguinal  ligament  now  comes  to  have 
its  origin  from  the  lower  pole  of  that  organ,  whence  it  extends  to 


360  THE    GENITAL    DUCTS 

the  anterior  abdominal  wall.  Owing  to  the  rudimentary  nature 
of  the  uterus  masculinus  and  the  sHght  development  of  its  walls 
the  inguinal  ligament  does  not  become  involved  with  it,  but  remains 
independent  and  forms  the  gubernaculum  testis  of  the  adult, 
whose  final  position  is  brought  about  by  the  descent  of  the  testis 
into  the  scrotum  (see  p.  369). 

In  the  Female. — In  the  female  the  transverse  partition  of  the 
pelvis  does  not  fuse  with  the  bladder  but  remains  distinct  as 
the  broad  ligament.  Consequently  there  is  in  the  female  both  a 
vesico-uterine  and  a  recto-uterine  pouch.  Since  the  genital  ridges 
form  upon  the  mesial  surfaces  of  the  Wolffian  ridges  and  the 
tubal  portions  are  their  lateral  portions,  when  these  latter  unite 
to  form  the  broad  ligament  the  ovary  will  come  to  lie  upon  the 
posterior  surface  of  that  structure,  projecting  into  the  recto- 
vesical pouch.  On  the  degeneration  of  the  mesonephros  the  peri- 
toneum that  covered  it  becomes  a  part  of  the  broad  ligament, 
forming  that  part  of  it  which  contains  the  Fallopian  tubes  and 
hence  is  known  as  the  mesosalpinx,  while  the  lower  part  of  the 
ligament,  on  account  of  its  relation  to  the  uterus,  is  termed  the 
mesometrium. 

The  genital  portion  of  the  mesonephros,  though  never  func- 
tional as  ducts  in  the  female,  persists  as  a  group  of  ten  to  fifteen 
tubules,  situated  between  the  two  layers  of  the  broad  ligament  and 
in  close  proximity  to  the  ovary;  these  consitute  what  is  known  as 
the  epoophoron  {parovarium  or  organ  of  Rosenmuller) .  The  tubules 
terminate  blindly  at  the  ends  nearest  the  ovary,  but  at  the  other 
extremity,  where  they  are  somewhat  coiled,  they  open  into  a 
collecting  duct  which  represents  the  upper  end  of  the  Wolffian 
duct.  Near  this  rudimentary  body  is  another  also  composed  of 
tubules,  representing  the  remains  of  the  excretory  portion  of  the 
mesonephros  and  termed  the  paroophoron  which,  however,  degen- 
erates during  the  early  years  of  extra-uterine  life.  So  far  as  the 
mesonephros  is  concerned,  therefore,  the  persisting  rudiments  in 
the  female  are  comparable  to  those  occurring  in  the  male. 

As  regards  the  ducts,  however,  the  case  is  different,  for  in  the 
female  it  is  the  Mlillerian  ducts  which  persist,  while  the  Wolffians 


THE    GENITAL   DUCTS 


361 


undergo  degeneration,  a  small  portion  of  their  upper  ends  per- 
sisting in  connection  with  the  epoophora,  while  their  lower  ends 
persist  as  straight  tubules  lying  at  the  sides  of  the  vagina  and  fornP 
ing  what  are  known  as  the  canals  of  Gartner.     The  Miillerian  ducts, 


UM 


Pig.  223. 


-Diagrams  Illustrating  the  Transformation  of  the   MtJLLERiAN 
AND  Wolffian  Ducts. 


B,  Bladder;  C,  clitoris;  CG,  canal  of  Gartner;  CI,  cloaca;  Eo,  epoophoron;  Ep, 
epididymis;  F,  Fallopian  tube;  G,  genital  gland;  HE,  hydatid  of  epididymis;  HM, 
hydatid  of  Morgagni;  K,  kidney;  MD,  Miillerian  duct;  O,  ovary;  P,  penis;  Po,  paro- 
ophoron; Pr,  prostate  gland;  R,  rectum;  T,  testis;  U,  urethra;  UM,  uterus  masculinus; 
Ur,  ureter;  US,  urogenital  sinus;  Ut,  uterus;  V,  vulva;  Va,  vas  aberrans;  VD,  vas 
deferens;  VS,  vesicula  seminalis;  WB,  Wolffian  body;  WD,  Wolffian  duct. — {Modi- 
fied from  Huxley.) 


on  the  other  hand,  become  converted  into  the  Fallopian  tubes 
{tubcB  uterince),  and  in  their  lower  portions  into  the  uterus  and  vag- 
ina. From  the  margins  of  the  openings  by  which  the  Miillerian 
ducts   communicate  with  the  coelom  projections  develop  at  an 


362  THE    GENITAL   DUCTS 

early  period  and  give  rise  to  the  fimbricB,  with  the  exception  of  the 
one  connected  with  the  ovary,  the  fimbria  ovarica,  which  is  the 
persisting  upper  portion  of  the  original  genital  ridge.  From  the 
utero-vaginal  canal  the  two  structures  which  give  it  its  name  are 
formed,  the  entire  canal  being  transformed  into  the  mucous  mem- 
brane of  the  uterus  and  vagina.  Indeed,  the  lower  ends  of  the 
Fallopian  tubes  are  also  taken  up  into  the  uterus,  for  the  conden- 
sation of  mesenchyme  that  takes  place  around  the  mucosa  to 
form  the  muscular  wall  of  the  uterus  is  so  voluminous  that  it  in- 
cludes not  only  the  utero-vaginal  canal  but  also  the  adjacent  por- 
tions of  the  MUllerian  ducts.  The  histological  differentiation 
of  the  uterus  from  the  vagina  begins  to  manifest  itself  at  about 
the  third  month,  and  during  the  fourth  month  the  vaginal  portion 
of  the  duct  becomes  flattened  and  the  epitheKum  lining  its  lumen 
fuses  so  as  to  completely  occlude  it  and,  a  little  later,  there  appears 
at  its  lower  opening  a  distinct  semicircular  fold.  This  is  the 
hymeUj  a  structure  which  seems  to  be  represented  in  the  male  by 
the  colliculus  seminalis.  The  obliteratron  of  the  lumen  of  the 
vagina  persists  until  about  the  sixth  month,  when  the  cavity  is 
re-estabhshed  by  the  breaking  down  of  the  central  epithelial  cells. 

The  extent  of  the  mesenchymal  condensation  to  form  the 
muscularis  uteri  also  produces  a  modification  of  the  relations  of 
the  inguinal  ligament  in  the  female.  For  the  ligament  becomes 
for  a  short  portion  of  its  length  included  in  the  condensation  and 
thus  attached  to  the  upper  portion  of  the  uterus.  It  is  conse- 
quently divided  into  two  portions,  one  extending  from  the  lower 
pole  of  the  ovary  to  the  uterus  and  forming  the  ligamentum  ovarii 
proprium  and  the  other  extending  from  the  uterus  to  the  anterior 
abdominal  wall  and  forming  what  is  known  in  the  adult  as  the 
round  ligament  of  the  uterus. 

The  diagram,  Fig.  223,  illustrates  the  transformation  from  the 
indifferent  condition  which  occurs  in  the  two  sexes,  and  that  the 
homologies  of  the  various  parts  may  be  clearly  understood  they 
may  also  be  stated  in  tabular  form  as  on  the  next  page. 

In  addition  to  the  sessile  hydatid,  a  stalked  hydatid  also  occurs  in 
connection  with  the  testis,  and  a  similar  structure  is  attached  to  the 


THE   BLADDER 


363 


fimbriated  opening  of  each  Fallopian  tube.  The  significance  of  these 
structures  is  uncertain,  though  it  has  been  suggested  that  they  are  per- 
sisting rudiments  of  the  pronephros.  

A  failure  of  the  development  of  the  various  parts  just  described  to  be 
completed  in  the  normal  manner  leads  to  various  abnormalities  in  con- 
nection with  the  reproductive  organs.  Thus  there  may  occur  a  failure 
in  the  fusion  of  the  lower  portions  of  the  MuUerian  ducts,  a  bihorned  or 
bipartite  uterus  resulting,  or  the  two  ducts  may  come  into  contact  and 
their  adjacent  walls  fail  to  disappear,  the  result  being  a  median  parti- 
tion separating  the  vagina  or  both  the  vagina  and  uterus  into  two 
compartments.  The  excessive  development  of  the  fold  which  gives 
rise  to  the  hymen  may  lead  to  a  complete  closure  of  the  lower  opening 
of  the  vagina,  while,  on  the  other  hand,  a  failure  of  the  Miillerian  ducts 
to  fuse  may  produce  a  biperf orate  hymen. 


Indifferent  Stage 

Male 

Female 

Genital  ridge 

Testis. 

Gubernaculum 

■! 

Fimbria  ovarica. 
Ovary, 

Ovarian  ligament. 
Round  ligament. 

f 
Wolfl&an  body 

Globus  major  of  epididymis. 

Paradidymis. 

Vasa  aberrantia. 

Epoophoron. 
Paroophoron. 

Wolffian  ducts 

Body  and  globus  minor 
epididymis. 
Vasa  deferentia. 
Seminal  vesicles. 
Ejaculatory  ducts. 

of 

Collecting  tubules  of  epo- 
ophoron. 

Canal  of  Gartner. 

Miillerian  ducts 

Sessile  hyatid. 
Uterus  masculinus. 

Fallopian  tubes. 

Uterus. 

Vagina. 

The  Development  of  the  Urinary  Bladder  and  the  Urogenital 
Sinus. — So  far  the  relations  of  the  lower  ends  of  the  urinogenital 
ducts  have  not  been  considered  in  detail,  although  it  has  been  seen 
that  in  the  early  stages  of  development  the  Wolffian  and  Miillerian 
ducts  open  into  the  sides  of  the  ventral  portion  of  the  cloaca; 
that  the  ureters  communicate  with  the  lower  portions  of  the  Wol- 
ffian ducts;  that  from  the  ventral  anterior  portion  of  the  cloaca  the 


364 


THE   BLADDER 


allantoic  duct  extends  outward  into  the  belly-stalk;  and  finally 
(p.  283),  that  the  cloaca  becomes  divided  into  a  dorsal  portion, 
which  forms  the  lower  part  of  the  rectum,  and  a  ventral  portion, 
which  is  continuous  with  the  allantois  and  receives  the  urinogenital 
ducts  (Fig.  224).  It  is  the  history  of  this  ventral  portion  of  the 
cloaca  which  is  now  to  be  considered. 

It  may  be  regarded  as  consisting  of  two  portions,  an  anterior 
and  a  posterior,  the  line  of  insertion  of  the  urinogenital  ducts 


Fig.  224.— Reconstruction  of  the  Cloacal  Region  of  an  Embryo  of  14  mm. 

al,  Allantois;  b,  bladder;  gt,  genital  tubercle;  i,  intestine;  n,  spinal  cord;  nc,  notochord; 

r,  rectum;  sg,  urogenital  sinus;  ur,  ureter;  w.  Wolffian  duct. — (Keibel.) 

marking  the  junction  of  the  two.  The  anterior  or  upper  portion 
is  destined  to  give  rise  to  the  urinary  bladder  (Fig.  224,  b),  while 
the  lower  one  forms  what  is  known  for  a  time  as  the  urogenital 
sinus  (sg).  The  bladder,  when  first  differentiated,  is  a  tubular 
structure,  whose  lumen  is  continuous  with  that  of  the  allantois, 
but  after  the  second  month  it  enlarges  to  become  more  sac-like, 
while  the  intraembryonic  portion  of  the  allantois  degenerates  to  a 
sohd  cord  extending  from  the  apex  of  the  bladder  to  the  umbilicus 
and  is  known  as  the  urachus.  During  the  enlargement  of  the 
bladder  the  terminal  portions  of  the  urinogenital  ducts  are  taken 


THE   BLADDER 


365 


up  into  its  walls,  a  process  which  continues  until  finally  the  ureters 
and  Wolffian  ducts  open  into  it  separately,  the  ureters  opening  to 
the  sides  of  and  a  Httle  anterior  to  the  ducts.  This  condition  is 
reached  in  embryos  of  about  14  mm.  (Fig.  224),  and  in  later 
stages  the  interval  between  the  two  pairs  of  ducts  is  increased 
(Fig.  225),  resulting  in  the  formation  of  a  short  canal  connecting 
the  lower  end  of  the  bladder  which  receives  the  ureters  with  the 
upper  end  of  the  urogenital  sinus,  into  which  the  Wolffian  and 


^^.. 


Fig.  225. — Reconstruction  of  the  Cloacal  Structures  of  an  Embryo  of  25  mm. 

bl.  Bladder;  m,  Miillerian  duct;  r,  rectum;  sg,  urogenital  sinus;  sy,  symphysis  pubis; 
u,  ureter;  ur,  urethra;  w.  Wolffian  duct. — (Adapted  from  Keibel.) 

Miillerian  ducts  open.  This  connecting  canal  represents  the 
urethra  (Fig.  225,  ur),  or  rather  the  entire  urethra  of  the  female 
and  the  proximal  part  of  that  of  the  male,  since  a  considerable 
portion  of  the  latter  canal  is  still  undeveloped  (see  p.  368).  From 
this  urethra  there  are  developed,  at  about  the  third  month,  a 
number  of  solid  outgrowths  which  represent  the  tubules  of  the 
prostate  gland  and  are  developed  in  both  sexes,  although  they  re- 
main in  a  somewhat  rudimentary '  condition  in  the  female.  In 
the  male  they  give  rise  to  somewhat  branched  tubular  glands  and 


366  THE    UROGENITAL   SINUS 

are  arranged  in  five  main  groups,  a  middle  group,  arising  from  the 
floor  of  the  urethra  above  the  entrance  of  the  ejaculatory  ducts, 
a  posterior  group  also  from  the  floor  but  below  the  openings  of 
the  ejaculatory  ducts,  two  lateral  groups  from  the  sides  of  the 
urethra  and  floor  of  the  grooves  on  either  side  of  the  colliculus 
seminalis,  and  an  anterior  group,  smaller  than  the  others,  from 
the  urethral  roof.  Each  of  these  groups  gives  rise  to  a  lobe  of 
the  prostate,  the  posterior  lobe  becoming  more  definitely  cir- 
cumscribed than  the  others  owing  to  the  formation  of  a  con- 
nective-tissue capsule  around  it.  A  few  scattered  outgrowths  arise 
from  the  floor  of  the  urethra  above  the  middle  group,  but  as  a 
rule  these  attain  only  a  sHght  development.  The  muscular  tissue, 
so  characteristic  of  the  gland  in  the  adult  male,  is  developed  from 
the  surrounding  mesenchyme  at  about  the  end  of  the  fourth  month. 

The  bladder  is,  accordingly,  essentially  a  derivative  of  the 
cloaca  and  its  mucous  membrane  is  therefore  largely  of  endodermal 
origin.  Portions  of  the  Wolffian  ducts,  which  are  of  mesodermal 
origin,  are,  however,  taken  up  into  the  wall  of  the  bladder  and  form 
a  portion  of  it.  The  extent  of  the  portion  so  formed  is  indicated 
by  the  position  of  the  orifices  of  the  ureters  above  and  of  the 
ejaculatory  ducts  below,  and  it  corresponds  therefore  with  what 
is  termed  the  trigonum  vesicce  together  with  the  floor  of  the  urethra 
as  far  as  the  openings  of  the  ejaculatory  ducts.  Throughout  this 
region  the  mucous  membrane  is  of  mesodermal  origin. 

The  urogenital  sinus  is  in  the  early  stages  also  tubular  in  its 
upper  part,  though  it  expands  considerably  below,  where  it  is 
closed  by  the  cloacal  membrane.  This,  by  the  separation  of  the 
cloaca  into  rectum  and  sinus,  has  become  divided  into  two  por- 
tions, the  more  ventral  of  which  closes  the  sinus  and  the  dorsal 
the  rectum,  the  interval  between  them  having  become  considerably 
thickened  to  form  the  perineal  body.  In  embryos  of  about  17 
mm.  the  urogenital  portion  of  the  membrane  has  broken  through, 
and  in  later  stages  the  tubular  portion  of  the  sinus  is  gradually 
taken  up  into  the  more  expanded  lower  portion,  until  finally  the 
entire  sinus  forms  a  shallow  depression,  termed  the  vestibule,  into 
the  upper  part  of  which  the  urethra  opens,  while  below  are  the 


THE    EXTERNAL   GENITALIA  367 

openings  of  the  Wolffian  (ejaculatory)  ducts  in  the  male  or  the 
orifice  of  the  vagina  in  the  female.  In  embryos  of  about  3.0  mm. 
vertex-breech  measurement  a  small  evagination  is  formed  on  each 
lateral  wall  of  the  sinus;  these  give  rise  to  the  bulbo-vestibular 
(Bartholin's)  glands  of  the  female  or  the  corresponding  bulbo- 
urethral glands  (Cowper's)  in  the  male. 

The  Development  of  the  External  Genitalia. — At  about  the 
fifth  week,  before  the  urogenital  sinus  has  opened  to  the  exterior, 
the  mesenchyme  on  its  ventral  wall  begins  to  thicken,  producing  a 
slight  projection  to  the  exterior.  This  eminence,  which  is  known 
as  the  genital  tubercle  (Fig.  224,  gt)^  rapidly  increases  in  size,  its 


Pig.  226. — The  External  Genitalia  of  an  Embryo  of  25  mm. 
a.  Anus;  gf,  genital  fold;  gl,  glans;  gs,  genital  swelling;  p,  perineal  body. — (Keibel.) 

extremity  becomes  somewhat  bulbously  enlarged  (Fig.  226,  gl) 
and  a  groove,  extending  to  the  base  of  the  terminal  enlargement, 
appears  upon  its  vestibular  surface,  the  lips  of  the  groove  forming 
two  well-marked  genital  folds  (Fig.  226,  gf).  At  about  the  tenth 
week  there  appears  on  either  side  of  the  tubercle  an  enlargement 
termed  the  genital  swelling  (Fig.  226,  gs),  which  is  due  to  a  thicken- 
ing of  the  mesenchyme  of  the  lower  part  of  the  ventral  abdominal 
wall  in  the  region  where  the  inguinal  ligament  is  attached,  and 
with  the  appearance  of  these  structures  the  indifferent  stage  of  the 
external  genitals  is  completed. 

In  the  female  the  growth  of  the  genital  tubercle  proceeds  rather 
slowly  and  it  becomes  transformed  into  the  clitoris,  the  genital  folds 


368 


THE   EXTERNAL    GENITALIA 


becoming  prolonged  to  form  the  labia  minora.  The  genital  swell- 
ings increase  in  size,  their  mesenchyme  becomes  transformed  into  a 
mass  of  adipose  and  fibrous  tissue  and  they  become  converted  into 
the  labia  majora,  the  interval  between  them  constituting  the  vulva. 

In  the  male  the  early  stages  of  development  are  closely  similar 
to  those  of  the  female;  indeed,  it  has  been  well  said  that  the 
external  genitals  of  the  adult  female  resemble  those  of  the  fetal 
male.  In  early  stages  the  genital  tubercle  elongates  to  form  the 
penis  and  the  integument  which  covers  the  proximal  part  of  it 
grows  forward  as  a  fold  which  encloses  the  bulbous  enlargement 
or  glans  and  forms  the  prepuce,  whose  epithelium  fuses  with  that 
covering  the  glans  and  only  separates  from  it  later  by  a  cornifica- 
tion  of  the  cells  along  the  plane  of  fusion.  The  genital  folds  meet 
together  and  fuse,  converting  the  vestibule  and  the  groove  upon 
the  vestibular  surface  of  the  penis  into  the  terminal  portion  of  the 
male  urethra  and  bringing  it  about  that  the  vasa  deferentia  and 
the  uterus  masculinus  open  upon  the  floor  of  that  passage.  The 
two  genital  swellings  are  at  the  same  time  brought  closer  together, 
so  as  to  lie  between  the  base  of  the  penis  and  the  perineal  body 
and,  eventually,  they  form  the  scrotum.  The  mesenchyme  of 
which  they  were  primarily  composed  differentiates  into  the  same 
layers  as  are  found  in  the  wall  of  the  abdomen  and  a  peritoneal  pouch 
is  prolonged  into  them  from  the  abdomen,  so  that  they  form  sacs 
Into  which  the  testes  descend  toward  the  close  of  fetal  life  (p.  370). 

The  homologies  of  the  portions  of  the  reproductive  apparatus 
derived  from  the  cloaca  and  of  the  external  genitalia  in  the  two 
sexes  may  be  perceived  from  the  following  table. 


Male 

Female 

Urinary  bladder. 

Urinary  bladder. 

Proximal  portion  of  urethra. 

Urethra. 

Bulbo-urethral  glands. 

Bulbo-vestibular  glands. 

Urogenital  sinus. . . 

The  rest  of  the  urethra. 

Vestibule. 

Genital  tubercle. . . 

Penis. 

Clitoris. 

Genital  folds 

Prepuce  and  integument  of  penis. 

Labia  minora. 

Genital  swellings. . 

Scrotum, 

Labia  majora 

THE   DESCENT    OF   THE    OVARIES  369 

Numerous  anomalies,  depending  upon  an  inhibition  or  excess  of  the 
development  of  the  parts,  may  occur  in  connection  with  the  external 
genitalia.  Should,  for  instance,  the  lips  of  the  groove  on  the  vestibu^;^ 
lar  surface  of  the  penis  fail  to  fuse,  the  penial  portion  of  the  urethra 
remains  incomplete,  constituting  a  condition  known  as  hypospadias,  a 
condition  which  offers  a  serious  bar  to  the  fulfilment  of  the  sexual  act. 
If  the  hypospadias  is  complete  and  there  be  at  the  same  time  an  im- 
perfect development  of  the  penis,  as  frequently  occurs  in  such  cases, 
the  male  genitalia  closely  resemble  those  of  the  female  and  a  condi- 
tion is  produced  which  is  usually  known  as  hermaphroditism.  It  is 
noteworthy  that  in  such  cases  there  is  frequently  a  somewhat  excessive 
development  of  the  uterus  masculinus,  and  a  similar  condition  may  be 
produced  in  the  female  by  an  excessive  development  of  the  clitoris. 
Such  cases,  however,  which  concern  only  the  accessory  organs  of  re- 
production, are  instances  of  what  is  more  properly  termed  spurious 
hermaphroditism,  true  hermaphroditism  being  a  term  which  should  be 
reserved  for  possible  cases  in  which  the  genital  ridges  give  rise  in  the 
same  individual  to  both  ova  and  spermatozoa.  Such  cases  are  of 
exceeding  rarity  in  the  human  species,  although  occasionally  observed 
in  the  lower  vertebrates,  and  the  great  majority  of  the  examples  of 
hermaphroditism  hitherto  observed  are  cases  of  the  spurious  variety. 

The  Descent  of  the  Ovaries  and  Testes. — The  positions 
finally  occupied  by  the  ovaries  and  testes  are  very  different  from 
those  which  they  possess  in  the  earlier  stages  of  development,  and 
this  is  especially  true  in  the  case  of  the  testes.  The  change  of 
position  is  partly  due  to  the  rate  of  growth  of  the  inguinal  liga- 
ments being  less  than  that  of  the  abdominal  walls,  the  reproductive 
organs  being  thereby  drawn  downward  toward  the  inguinal 
regions  where  the  ligaments  are  attached.  The  point  of  attach- 
ment is  beneath  the  bottom  of  a  slight  pouch  of  peritoneum 
which  projects  a  short  distance  into  the  substance  of  the  genital 
swellings  and  is  known  as  the  canal  of  Nuck  in  the  female,  and  in 
the  male  as  the  vaginal  process. 

In  the  female  a  second  factor  combines  with  that  just  men- 
tioned. The  relative  shortening  of  the  inguinal  ligaments  acting 
alone  would  draw  the  ovaries  to-ward  the  inguinal  regions,  but 
since  they  are  united  to  the  uterus  by  the  ovarian  ligaments  move- 
ment in  that  direction  is  prevented  and  the  ovaries  come  to  lie  in 
the  recto-uterine  compartment  of  the  pelvic  cavity. 

With  the  testes  the  case  is  more  complicated,  since  in  addition 

24 


370 


THE  DESCENT  OF  THE  TESTES 


to  the  relative  shortening  of  the  inguinal  ligaments  there  is  an 
elongation  of  the  vaginal  processes  into  the  substance  of  the  genital 
swellings,  and  it  must  be  remembered  that  the  testes,  like  the 
ovaries,  are  primarily  connected  with  the  peritoneum.  Three 
stages  may  be  recognized  in  the  descent  of  the  testes.  The 
first  of  these  depends  on  the  slow  rate  of  elongation  of  the  inguinal 
ligaments  or  gubernacula.  It  lasts  until  about  the  fifth  month 
of  development,  when  the  testes  lie  in  the  inguinal  region  of  the 
abdomen,  but  during  this  month  the  elongation  of  the  gubernacu- 
lum  becomes  more  rapid  and  brings  about  the  second  stage,  dur- 
ing which  there  is  a  slight  ascent  of  the  testes,  so  that  they  come 


Fig.  227. — Diagrams  Illustrating  the  Descent  of  the  Testis. 
il.  Inguinal  ligament;  m,  muscular  layer;  s,  skin  and  dartos  of  the  scrotum;  t,  testis; 
tv,  tunica  vaginalis;  vd,  vas  deferens;  vp,  vaginal  process  of  peritoneum. — (After 
Her  twig.) 

to  lie  a  little  higher  in  the  abdomen.  This  stage  is,  however,  of 
short  duration,  and  is  succeeded  by  the  stage  of  the  final  descent, 
which  is  characterized  by  the  elongation  of  the  vaginal  processes 
of  the  peritoneum  into  the  substance  of  the  scrotum  (Fig.  227,  ^4). 
Since  the  gubernaculum  is  attached  to  the  abdominal  wall  be- 
neath this  process,  and  since  its  growth  has  again  diminished,  the 
testes  gradually  assume  again  their  inguinal  position,  and  are 
finally  drawn  down  into  the  scrotum  with  the  vaginal  processes. 

The  condition  which  is  thus  acquired  persists  for  some  time 
after  birth,  the  testicles  being  readily  pushed  upward  into  the 
abdominal  cavity  along  the  cavity  by  which   they  descended. 


LITERATURE  371 

Later,  however,  the  size  of  the  openings  of  the  vaginal  processes 
into  the  general  peritoneal  cavity  becomes  greatly  reduced,  so 
that  each  process  becomes  converted  into  an  upper  narrow  neckT 
and  a  lower  sac-like  cavity  (Fig.  227,  B),  and,  still  later,  the  walls 
of  the  neck  portion  fuse  and  become  converted  into  a  solid  cord, 
while  the  lower  portion,  wrapping  itself  around  the  testis,  becomes 
the  tunica  vaginalis  (tv).  By  these  changes  the  testes  become 
permanently  located  in  the  scrotum.  During  the  descent  of 
the  testes  the  remains  of  each  Wolffian  body,  the  epididymis,  and 
the  upper  part  of  each  vas  deferens  together  with  the  spermatic 
vessels  and  nerves,  are  drawn  down  into  the  scrotum,  and  the 
mesenterial  fold  in  which  they  were  originally  contained  also 
practically  disappears,  becoming  converted  into  a  sheath  of  .con- 
nective tissue  which  encloses  the  vas  deferens  and  the  vessels  and 
nerves,  binding  them  together  into  what  is  termed  the  spermatic 
cord.  The  mesorchium,  which  united  the  testis  to  the  peritoneum 
enclosing  the  Wolffian  body,  does  not  share  in  the  degeneration  of 
the  latter,  but  persists  as  a  fold  extending  between  the  epididymis 
and  the  testis  and  forming  the  sinus  epididymidis. 

In  the  text-books  of  anatomy  the  spermatic  cord  is  usually  described 
as  lying  in  an  inguinal  canal  which  traverses  the  abdominal  walls  ob- 
liquely immediately  above  Poupart's  ligament.  So  long  as  the  lumen 
of  the  neck  portion  of  the  vaginal  process  of  peritoneum  remains 
patent  there  is  such  a  canal,  placing  the  cavity  of  the  tunica  vaginalis 
in  communication  with  the  general  peritoneal  cavity,  but  the  cord 
does  not  traverse  this  canal,  but  lies  outside  it  in  the  retroperitoneal 
connective  tissue.  When,  however,  the  neck  of  the  vaginal  process 
disappears,  a  canal  no  longer  exists,  although  the  connective  ti^ue 
which  surrounds  the  spermatic  cord  and  unites  it  with  the  tissues  of 
the  abdominal  walls  is  less  dense  than  the  neighboring  tissues,  so  that 
the  cord  may  be  readily  separated  from  these  and  thus  appear  to  lie  in 
a  canal. 

LITERATURE 

B.  M.  Allen  :  "  The  Embryonic  Development  of  the  Ovary  and  Testes  in  Mammals," 
Amer.  Journ.  of  Anat.,  iii,  1904. 

J.  L.  Bremer:  "Morphology  of  the  Tubules  of  the  Human  Testis  and  Epididymis," 
Amer.  Journ.  Anat.,  xi,  191 1. 

A.  H.  Eggerth:  "On  the  anlage  of  the  bulbo-urethral  (Cowper's)  and  major  vestib- 
ular (Bartholin's)  glands  in  the  human  embryo,"  Anat.  Record,  ix,  1915. 


372  LITERATURE 

E.  J.  Evatt:  "A  Contribution  to  the  Development  of  the  Prostate  in  Man,"  Jotirn. 

Anat.  and  Phys.,  xliii,  1909. 
E.  J.  Evatt:  "A  Contribution  to  the  Development  of  the  Prostate  Gland  in  the 

Human  Female,"  Journ.  Anat.  and  Phys.,  xlv,  191  i. 
W.  Felix:  "Die  Entwichlung  der  Harn-  und  Geschlechtsorgane/'  in  Keibel-Mall 

Human  Embryology,  it,  1912. 
A.  Fleischmann:  "  Morphologische  Studien  iiber  Kloake  und  Phallus  der  Amnioten, 

Morphol.  Jahrhuch,  xxx,  xxxii,  und  xxxvi,  1902,  1904,  1907. 
O.  Frankl:  "Beitrage  zur  Lehre  vom  Descensus  te%iic\x\ov\im"  Sitzimgsher.  der 

kais.  Akad.  Wissensch.  Wien,  Math.-Naturwiss.  Classe,  cix,  1900. 
A.  Fuss:  "Ueber  die  Geschlechtzelle  des  Menschen  und  die  Saugetiere,"  Arch. 

fiir  mikrosk.  Anat.,  lxxxi,  191 2. 
S.  P.  Gage:  "A  Three  Weeks  Human  Embryo,  with  especial  reference  to  the  Brain 

and  the  Nephric  System,"  Amer.  Journ.  of  Anat.,  iv,  1905. 
D.  B.  Hart:  "The  Nature  and  Cause  of  the  Physiological  Descent  of  the  Testes," 

Journ.  Anat.  and  Phys.  xliv,  1909. 

D.  B-  Hart:  "The  Physiological  Descent  of  the  Ovaries  in  the  Human  Foetus," 

Journ.  Anat.  and  Phys.,  xliv,  1909. 

E.  Hauch:  "Ueber  die  Anatomie  und  Entwicklung  der  Nieren."  Anat.  Hefte,  xxii, 

1903. 
G.  C.  HuBER :  "On  the  Development  and  Shape  of  the  Urinif erous  Tubules  of  Certain 

of  the  Higher  Mammals,"  Amer.  Journ.  of  Anat.,  iv,  Suppl.,  1905. 
J.  Janosik:  "  Histologisch-embryologische  Untersuchungen  iiber  das  Urogenital- 

system,"  Sitzungsher.  der  kais.  Akad.  Wissensch.  Wien,  Math.-Naturwiss.  Classe, 

xci,  1887. 
J.  Janosik:  "Ueber  die  Entwicklung  der  Nachniere  bei  den  Amnioten,"  Arch,  fiir 

Anat.  u.  Phys.,  Anat.  Ahth.,  1907. 
J.  Janosik:  "Entwicklung  des  Nierenbeckens  beim  Menschen,"  Arch,  fur  mikrosk 

Anat.,  LxxiTi,  1911. 

F.  Ketbel:  "Zur  Entwickelungsgeschichte  des  menschlichen  Urogenital-apparatus," 

Archiv  fiir  Anat.  und  Physiol.,  Anat.  Ahth.,  1896. 
O.  S.  Lowsley:  "The  development  of  the  human  prostate  gland,  etc.,"  Amer. 

Journ.  Anat.,  xiii,  191 2. 
J.  B.  Macallum:  "Notes  on  the  Wolffian  Body  of  Higher  Mammals,"  Amer.  Journ. 

tAnat.,  I,  1902. 
E.  Martin:  "Ueber  die  Anlage  der  Urniere  beim  Kaninchen,"  Archiv  fiir  Anat.  und 

Physiol.,  Anat.  Ahth.,  1888. 
H.  Meyer:  "Die  Entwickelung  der  Urnieren  beim  Menschen,"  Archiv  fiir  mikrosk. 

Anat.,  xxxvi,  1890. 
R.  Meyer:  "Zur  Kenntnis  des  Gartner'schen  Ganges  besonders  in  der  Vagina  und 

dem  Hymen  des  Menschen,"  Arch,  fiir  mikrosk.  Anat.,  lxxiii,  1909. 
R.  Meyer:  "Zur  Entwicklungsgeschichte  und  Anatomie  des  utriculus  prostaticus 

beim  Menschen,"  Arch,  fiir  mikrosk.  Anat.,  lxxiv,  1909. 

G.  von   Mihalkovicz:  "Untersuchungen   iiber  die  Entwickelung  des  Harn-  und 

Geschlechtsapparates  der  Amnioten,"  Internat.  Monatsschrift  fiir  Anat.  und 
Physiol.,  II,  1885. 


LITERATURE  373 

W.  Nagel:  "Ueber  die  Entwickelung  des  Urogenitalsystems  des  Menschen,"  Archiv 

fiir  mikros.  Anat.,  xxxrv,  1889. 
W.  Nagel:  "Ueber  die  Entwicklung  des  Uterus  und  der  Vagina  beim  Menschen," 

Archiv  fiir  mikrosk.  Anat.,  xxxvii,  1891. 
W.  Nagel:  "Ueber     die   Entwickelung  der  innere  und  aussere  Genitalien  beim 

menschlichen  Weibes,"  Archiv  fUr  GynakoL,  xlv,  1894. 
G.  Pallin:  "Beitrag  zur  Anatomie  und  Embryologie  der  Prostata  und  der  Samen- 

blasen,"  Arch,  fiir  Anat.  und  Physiol. ,  Anat.  Abth.,  1901. 
K.  Peter:  " Untersuchungen  iiber  Bau  und  Entwicklung  der  Niererl.  Die  Nieren- 

kanalchen  des  Menschen  und  einiger  Saugetiere,  Jena,  1909. 
A.  G.  Pohlman:  "The  Development  of  the  Cloaca  in  Human  Embryos,"  Amer. 

Joiirn.  of  Anat.,  xii,  191 1. 
W.   Rubaschkin:  "Ueber  die  Urgeschlechtszellen  bei  Saugetiere,"   Anat.   Hefte, 

xxxEX,  1909. 
K.  E.  Schreiner:  "Ueber  die  Entwicklung  der  Amniotenhiere,"  Zeit.fiir  wissensch. 

ZooL,  Lxxi,  1902. 
O.  Stoerk:  "Beitrag  zur  Kenntnis  des  Aufbaues  der  menschlichen  Niere,"  Anat. 

Hefte,  xxm,  1904. 
J.  Tandler:  "Ueber  Vornieren-Rudimente  beim  menschliche  Embryo,"  Anat.  Hefte, 

XXVIII,  1905. 
F.  J.  Taussig:  "The  Development  of  the  Hymen,"  Amer.  Journ.  Anat.,  viii,  1908. 
F.   Tourneux:  "Sur   le   developpement  et  revolution  du  tubercle  genital  chez  le 

foetus  humain  dans  les  deux  sexes,"  Journ.  de  VAnat  et  de  Physiol.,  xxv,  1889. 
E.  M.  Watson:  "The  Development  of  the  Seminal  Vesicles  in  Man,"  Amer.  Journ. 

Anat.,  xxrv,  1918. 
S.  Weber:  "Zur  Entwickelungsgeschichte  des  uropoetischen  Apparates  bei  Saugern, 

mit  besonderer  Beriicksichtigung  der  Urniere  zur  Zeit  des  Auftretens  der  blei- 

benden  Niere,"  Morphol.  Arbeiten,  vii,  1897. 


CHAPTER  XIV 
THE  SUPRARENAL  SYSTEM  OF  ORGANS 

To  the  suprarenal  system  a  number  of  bodies  of  peculiar  struc- 
ture, probably  concerned  with  internal  secretion,  may  be  assigned. 
In  the  fishes  they  fall  into  two  distinct  groups,  the  one  contain- 
ing organs  derived  from  the  ccelomic  epithelium  and  known  as 
interrenal  organs,  and  the  other  consisting  of  organs  derived  from 
the  sympathetic  nervous  system  and  which,  on  account  of  the 
characteristic  affinity  they  possess  for  chromium  salts,  have  been 
termed  the  chromaffine  organs.  But  in  the  amphibia  and  amniote 
vertebrates,  while  both  the  groups  are  represented  by  independent 
organs,  yet  they  also  become  intimately  associated  to  form  the 
suprarenal  bodies,  so  that,  notwithstanding  their  distinctly  dif- 
ferent origins,  it  is  convenient  to  consider  them  together. 

The  Development  of  the  Suprarenal  Bodies.^ — The  supra- 
renal bodies  make  their  appearance  at  an  early  stage,  while  the 
Wolffian  bodies  are  still  in  a  well-developed  condition,  and  they 
are  situated  at  first  to  the  medial  side  of  the  upper  ends  of  these 
structures  (Fig.  216,  sr).  Their  final  relation  to  the  metanephros 
is  a  secondary  event,  and  is  merely  a  topographic  relation,  there 
being  no  developmental  connection  between  the  two  structures. 

In  the  human  embryo  they  make  their  appearance  at  about 
the  beginning  of  the  fourth  week  of  development  as  a  number  of 
proliferations  of  the  ccelomic  epithelium,  which  project  into  the 
subjacent  mesenchyme,  and  are  situated  on  either  side  of  the 
median  line  between  the  root  of  the  mesentery  and  the  upper  por- 
tion of  the  Wolffian  body.  The  various  proliferations  soon  sepa- 
rate from  the  epithelium  and  unite  to  form  two  masses  situated 
in  the  mesenchyme,  one  on  either  side  of  the  upper  portion  of 
the  abdominal  aorta.  In  certain  forms,  such  as  the  rabbit,  the 
primary  proliferations  arise  from  the  bottom  of  depressions  of  the 

374 


DEVELOPMENT  OF  THE  SUPRARENAL  BODIES       375 

coelomic  epithelium  (Fig.  228),  but  in  the  human  embryo  these 
depressions  do  not  form. 

Up  to  this  stage  the  structure  is  a  pure  interrenal  organ,  but" 
during  the  fifth  week  of  development  masses  of  cells,  derived  from 
the  abdominal  portion  of  the  sympathetic  nervous  system,  begin 
to  penetrate  into  each  of  the  interrenal  masses  (Fig.  229),  and  form 
strands  traversing  them.  At  about  the  ninth  or  tenth  week  fatty 
granules  begin  to  appear  in  the  interrenal  cells  and  somewhat 
later,  about  the  fourth  month,  the  sympathetic  constituents  begin 
to  show  their  chromaffine  characteristics.  The  two  tissues,  how- 
ever, remain  intermingled  for  a  considerable  time,  and  it  is  not 


/ 


wc 


\x 


tw"  '  ■'o^^' 


Ao  I    M 


re  "  '"-. c::^.-'^  ■■,  A 


Fig.  228. — Section  through  a  Portion  of  the  Wolffian  Ridge  of  a  Rabbit 

Embryo  of  6.5  mm. 

Ao,  Aorta;  ns,  nephrostome;  Sr,  suprarenal  body;  vc,  cardinal  vein;  wc,  tubule  of 

Wolffian  body;  wd.  Wolffian  duct. — (Aichel.) 

until  a  much  later  period  that  they  become  definitely  separated, 
the  sympathetic  elements  gradually  concentrating  in  the  center  of 
the  compound  organ  to  become  its  medullary  substauce,  while  the 
interrenal  tissue  forms  the  cortical  substance.  Indeed,  it  is  not 
until  after  birth  that  the  separation  of  the  two  tissues  and  their 
histological  differentiation  is  complete,  occasional  masses  of  inter- 
renal tissue  remaining  imbedded  in  the  medullary  substance  and 
an  immigration  of  sympathetic  cells  continuing  until  at  least  the 
tenth  year  (Wiesel). 

A  great  deal  of  difference  of  opinion  has  existed  in  the  past  con- 
cerning the  origin  of  the  suprarenal  glands.  By  several  authors  they 
have  been  regarded  as  derivatives  in  whole  or  in  part  of  the  excretory 
apparatus,  some  tracing  their  origin  to  the  mesonephros  and  others 
even  to  the  pronephros.     The  fact  that  in  some  mammals  the  cortical 


3^6       DEVELOPMENT  OF  THE  SUPRARENAL  BODIES 

(interrenal)  cells  are  formed  from  the  bottom  of  depressions  of  the 
coelomic  epithelium  seemed  to  lend  support  to  this  view,  but  it  is  now- 
pretty  firmly  established  that  the  appearances  thus  presented  do  not 
warrant  the  interpretation  placed  upon  them  and  that  the  interrenal 
tissue  is  derived  from  the  coelomic  epithelium  quite  independently  of 
the  nephric  tubules.  That  the  chromaffine  tissue  is  a  derivative  of  the 
sympathetic  nervous  system  has  long  been  recognized. 

During  the  development  of  the  suprarenal  glands  portions  of 
their  tissue  may  be  separated  as  the  result  of  unequal  growth  and 
form  what  are  commonly  spoken  of  as  accessory  suprarenal  glands, 
although,  since  they  are  usually  composed  solely  of  cortical  sub- 
stance, the  term  accessory  interrenal  bodies  would  be  more  ap- 

S.  SB. 


S.B. 

Fig.  229. — Section  through  the  Suprarenal  Body  of  an  Embryo  of  17  mm. 
A ,  Aorta;  R,  interrenal  portion;  5,  sympathetic  nervous  system;  SB,  sympathetic  cells. 
>     penetrating  the  interrenal  portion. — (Wiesel.) 

propriate.  They  may  be  formed  at  different  periods  of  develop- 
ment and  occur  in  various  situations,  as  for  instance,  in  the  vicinity 
of  the  kidneys  or  even  actually  imbedded  in  their  substance,  on 
the  walls  of  neighboring  blood-vessels,  in  the  retroperitoneal  tissue 
below  the  level  of  the  kidneys,  and  in  connection  with  the  organs 
of  reproduction,  in  the  spermatic  cord,  epididymis  or  rete  testis 
of  the  male  and  in  the  broad  ligament  of  the  female. 

It  seems  probable  that  the  bodies  associated  with  the  repro- 
ductive apparatus  are  separated  from  the  main  mass  of  interrenal 
tissue  before  the  immigration  of  the  sympathetic  tissue  and  before 


DEVELOPMENT  OF  THE  SUPRARENAL  BODIES 


377 


the  descent  of  the  ovaries  or  testes,  while  those  which  occur  at 
higher  levels  are  of  .later  origin,  and  in  some  cases  may  contain 
some  medullary  substance,  being  then  true  accessory  suprarenals- 
Such  bodies  are,  however,  comparatively  rare,  the  great  majority 
of  the  accessory  bodies  being  composed  of  interrenal  tissue  alone. 
Independent  chromaffine  organs  also  occur,  among  them  the 
intercarotid  ganglia  and  the  organs  of  Zuckerkandl  being  especially 


Fig.  230. — Section  of  a  Cell  Ball  from  the  Intercarotid  Ganglion  of  Man. 
be,  Blood  capillaries;  ev,  efferent  vein;  S,  connective-tissue  septum;'/,  trabeculae. — 
{From  Bohm  and  Davidoff,  after  Schaper.) 

deserving  of  note.  It  may  also  be  pointed  out,  however,  that  the 
chromaffine  cells  have  the  same  origin  as  the  cells  of  the  sympa- 
thetic ganglia  and  may  sometimes  fail  to  separate  from  the  latter 
so  that  the  sympathetic  ganglia  and  plexuses  frequently  contain 
chromafhne  cells. 

The  Intercarotid  Ganglia. — These  structures,  which  are  fre- 
quently though  incorrectly  termed  carotid  glands,  are  small  bodies 
about  5  mm.  in  length,  which  lie  usually  to  the  mesial  side  of 
the  upper  ends  of  the  common  carotid  arteries.  They  possess  a 
very  rich  arterial  supply  and  stand  in  intimate  relation  with  the 


378  THE   INTERCAROTID    GANGLIA 

branches  of  an  intercarotid  sympathetic  plexus,  and,  furthermore, 
they  are  characterized  by  possessing  as  their  specific  constituents 
markedly  chromaffine  cells,  among  which  are  scattered  stellate 
cells  resembling  the  cells  of  the  sympathetic  ganglia. 

They  have  been  found  to  rise  in  pig  embryos  of  44  mm.  by  the 
separation  of  cells  from  the  ganglionic  masses  scattered  through- 
out the  carotid  sympathetic  plexuses.  These  cells,  which  become 
the  chromafhne  cells,  arrange  themselves  in  round  masses  termed 
cell  balls,  many  of  which  unite  to  form  each  ganglion,  and  in  man 
each  cell  ball  becomes  broken  up  into  trabecules  by  the  blood- 
vessels (Fig.  230)  which  penetrate  its  substance,  and  the  individual 
balls  are  separated  from  one  another  by  considerable  quantities 
of  connective  tissue. 

Some  confusion  has  existed  in  the  past  as  to  the  origin  of  this 
structure.  The  mesial  wall  of  the  proximal  part  of  the  internal  carotid 
artery  becomes  considerably  thickened  during  the  early  stages  of 
development  and  the  thickening  is  traversed  by  numerous  blood 
lacunae  which  communicate  with  the  lumen  of  the  vessel.  This  condi- 
tion is  perhaps  a  relic  of  the  branchial  capillaries  which  in  the  lower  gill- 
breathing  vertebrates  represent  the  proximal  portion  of  the  internal 
carotid,  and  has  nothing  to  do  with  the  formation  of  the  intercarotid 
ganglion,  although  it  has  been  believed  by  some  authors  (Schaper) 
that  the  ganglion  was  derived  from  the  thickening  of  the  wall  of  the 
vessel.  The  fact  that  in  some  animals,  such  as  the  rat  and  the  dog, 
the  ganglion  stands  in  relation  with  the  external  carotid  and  receives 
its  blood-supply  from  that  vessel  is  of  importance  in  this  connection. 

The  thickening  of  the  internal  carotid  disappears  in  the  higher 
vertebrates  almost  entirely,  but  in  the  Amphibia  it  persists  throughout 
life,  the  lumen  of  the  proximal  part  of  the  vessel  being  converted  into  a 
fine  mesh  work  by  the  numerous  trabeculae  which  traverse  it.  This 
carotid  labyrinth  has  been  termed  the  carotid  gland,  a  circumstance 
which  has  probably  assisted  in  producing  confusion  as  to  the  real 
significance  of  the  intercarotid  ganglion. 

The  Organs  of  Zuckerkandl. — In  embryos  of  14.5  mm.  there 
have  been  found,  in  front  of  the  abdominal  aorta,  closely  packed 
groups  of  cells  which  resemble  in  appearance  the  cells  composing 
the  ganglionated  cord,  two  of  these  groups,  which  extend  down- 
ward along  the  side  of  the  aorta  to  below  the  point  of  origin  of  the 
inferior  mesenteric  artery,  being  especially  distinct.  These  cell 
groups  give  rise  to  the  ganglia  of  the  praevertebral  sympathetic 


THE    ORGANS    OF    ZUCKERKANDL 


379 


plexuses  and  also  to  peculiar  bodies  which,  from  their  discoverer, 
may  be  termed  the  organs  of  Zuckerkandl.  Each  body  stands  in_ 
intimate  relation  with  the  fibers  of  the  sympathetic  plexuses  and 
has  a  rich  blood-supply,  resembling  in  these  respects  the  inter- 
carotid  ganglia,  and  the  resemblance  is  further  increased  by  the 
fact  that  the  specific  cells  of  the  organ  are  markedly  chromaffine. 


n.r. 


I  Fig.  231. — Organs  of  Zuckerkandl  from  a  New-born  Child. 

a.  Aorta;  ci,  inferior  vena  cava;  i.c,  common  iliac  artery;  mi,  inferior  mesenteric 
artery;  n.l  and  n.r,  left  and  right  accessory  organs;  pi.  a,  aortic  plexus;  u,  ureter;  v.r.s, 
left  renal  vein. — {Zuckerkandl.) 

At  birth  the  bodies  situated  in  the  upper  portion  of  the  ab- 
dominal cavity  have  broken  up  into  small  masses,  but  the  two 
lower  ones,  mentioned  above,  are  still  well  defined  (Fig.  231). 
Even  these,  however,  seem  to  disappear  later  on  and  no  traces  of 
them  have  as  yet  been  found  in  the  adult. 


380  LITERATURE 


LITERATURE 

A.  Kohn:  "Ueber  den  Bau  und  die  Entwickelung  der  sog.  Carotisdriise,"  Archiv 

fur  mikrosk.  Anat.,  lvi,  1900. 
A.  Kohn:  "Das  chromaffine  Gewebe,"  Ergebn.  der  Anal,  und  Entwickelungsgesch., 

XII,  1902. 
H.  Poll:  "Die  vergleichende  Entwicklungsgeschichte  der  Nebennierensysteme  der 

Wirbeltiere,"    Hertwig^s   Handb.   der  vergl.   und  exper.   Entwicklungslehre  der 

Wirbeltiere,  iii,  1906. 
A.   SouLiifi:  "Recherches  sur  le  developpement  des  capsules  surrenales  chez  les 

Vert^bres,"  Journ.  de  I' Anat.  et  de  la  Physiol.,  xsxix,  1903. 
J.  Wiesel:  "Beitrage  zur  Anatomie  und  Entwickelung  der  menschlichen  Neben- 

niere,"  Anat.  Heft.,  xtk,  1902. 
E.  Zuckerkandl:  "Ueber  Nebenorgane  des  Sympathicus  im  Retroperitonealraum 

des  Menschen,"  Verhandl.  Anat.  Gesellsch.,  xv,  1901. 


CHAPTER  XV 
THE  DEVELOPMENT  OF  THE  NERVOUS  SYSTEM 

The  Histogenesis  of  the  Nervous  System. — The  entire  central 
nervous  system  is  derived  from  the  cells  lining  the  medullary 
groove,  whose  formation  and  conversion  into  the  medullary  canal 
has  already  been  described  (p.  76).  When  the  groove  is  first 
formed,  the  cells  lining  it  are  somewhat  more  columnar  in  shape 
than  those  on  either  side  of  it,  though  like  them  they  are  arranged 
in  a  single  layer;  later  they  increase  by  mitotic  division  and  ar- 
range themselves  in  several  layers,  so  that  the  ectoderm  of  the 
groove  becomes  very  much  thicker  than  that  of  the  general  surface 
of  the  body.  At  the  same  time  the  cell  boundaries,  which  were 
originally  quite  distinct,  gradually  disappear,  the  tissue  becoming 
a  syncytium.  While  its  tissue  is  in  this  condition  the  lips  of  the 
medullary  groove  unite,  and  the  subsequent  differentiation  of  the 
canal  so  formed  differs  somewhat  in  different  regions,  although  a 
fundamental  plan  may  be  recognized.  This  plan  is  most  readily 
perceived  in  the  region  which  becomes  the  spinal  cord,  and  may  be 
described  as  seen  in  that  region. 

Throughout  the  earlier  stages,  the  cells  lining  the  inner  wall  of 
the  medullary  tube  are  found  in  active  proliferation,  some  of  the 
cells  so  produced  arranging  themselves  with  their  long  axes  at 
right  angles  to  the  central  canal  (Fig.  232),  while  others,  whose  des- 
tiny is  for  the  most  part  not  yet  determinable  and  which  therefore 
may  be  termed  indifferent  cells,  are  scattered  throughout  the  syn- 
cytium. At  this  stage  a  transverse  section  of  the  medullary  tube 
shows  it  to  be  composed  of  two  well-defined  zones,  an  inner  one 
immediately  surrounding  the  central  canal  and  composed  of  the 
indifferent  cells  and  the  bodies  of  the  inner  or  ependymal  cells ^  and 
an  outer  one  consisting  of  branched  prolongations  of  the  syncytial 
cytoplasm.     This  outer  layer  is  termed  the  marginal  velum  (Rand- 

381 


382 


THE   HISTOGENESIS    OF   THE    NERVOUS    SYSTEM 


schleier)  (Fig.  232,  w).  The  indifferent  cells  now  begin  to  wander 
outward  to  form  a  definite  layer,  termed  the  mantle  layer,  lying  be- 
tween the  marginal  velum  and  the  bodies  of  the  ependymal  cells 
(Fig.  233),  and  when  this  layer  has  become  well  established  the 
cells  composing  it  begin  to  divide  and  to  differentiate  into  (i)  cells 
termed  neuroblasts,  destined  to  become  nerve-cells,  and  (2)  others 


'?':;''SI' 


mv 


y^--;.. 


cs 


Fig.  232. — Transverse  Section  through  the  Spinal  Cord  of  a  Pig  Embryo 
OF  30  MM.,  THE  Upper  Part  Showing  the  Appearance  Produced  by  the  Silver. 
Method  of  Demonstrating  the  Neuroglia  Fibers. 

a,  Ependyma  of  floor  plate;  &,  boundary  between  mantle  layer  and  marginal 
zone;  cs,  mesenchymal  connective- tissue  syncytium;  ep,  ependymal  cells;  i,  ingrowth 
of  connective  tissue;  m,  marginal  velum;  mm,  mantle  layer;  mv,  mantle  layer  of  floor 
plate;  p,  pia  mater;  r,  neuroglia  fibers. — (Hardesty.) 

which  appear  to  be  supportive  in  character  and  are  termed  neuro- 
glia cells  (Fig.  233,  B).  The  latter  are  for  the  most  part  small  and 
are  scattered  among  the  neuroblasts,  these,  on  the  other  hand, 
being  larger  and  each  early  developing  a  single  strong  process 
which  grows  out  into  the  marginal  velum  and  is  known  as  an  axis- 
cylinder.  At  a  later  period  the  neuroblasts  also  give  rise  to  other 
processes,  termed  dendrites,  more  slender  and  shorter  than  th( 


THE    HISTOGENESIS    OF    THE    NERVOUS    SYSTEM 


3^3 


axis-cylinders,  branching  repeatedly,  and,  as  a  rule,  not  extending 
beyond  the  limits  of  the  mantle  layer. 

In  connection  with  the  neuroglia  cells  peculiar  neuroglia  fibrils 
develop  very  much  in  the  same  way  as  the  fibers  are  formed  in 
mesenchymal  connective  tissue.  That  is  to  say,  they  are  formed 
from  the  peripheral  portions  of  the  cytoplasm  of  the  neuroglial 
and  ependymal  cells.  But  since  these  cells  are  connected  to- 
gether to  form  a  syncytium  the  fibrils  are  not  confined  to  the 


'O^O 


Fig.  233. — Diagram  showing  the  Development  of  the  Mantle  Layer  in  the 

Spinal  Cord. 
The  circles,  indifferent  cells;  circles  with  dots,  neuroglia  cells;  shaded  cells,  ger- 
minal cells;  circles  with  cross,  germinal  cells  in  mitosis;  black  cells,  nerve-cells. — 
(Schaper.) 

territories  of  the  individual  cells,  but  may  extend  far  beyond  these, 
passing  in  the  syncytium  from  the  territory  of  one  neuroglial 
cell  to  another,  many  of  those,  indeed,  arising  in  connection  with 
the  ependymal  cells  extending  throughout  the  entire  thickness  of 
the  medullary  wall  (Fig.  231).  The  fibrils  branch  abundantly 
and  form  a  supportive  network  extending  through  all  portions 
of  the  central  nervous  system. 

The  axis-cylinder  processes  of  the  majority  of  the  neuroblasts 
on  reaching  the  marginal  velum  bend  upward  or  downward  and, 
after  traversing  a  greater  or  less  length  of  the  cord,  re-enter  the 


384 


THE    HISTOGENESIS    OF    THE    NERVOUS    SYSTEM 


mantle  layer  and  terminate  by  dividing  into  numerous  short 
branches  which  come  into  relation  with  the  dendrites  of  adjacent 
neuroblasts.  The  processes  of  certain  cells  situated  in  the  ventral 
region  of  the  mantle  zone  pass,  however,  directly  through  the 
marginal  velum  out  into  the  surrounding  tissues  and  constitute 
the  ventral  nerve-roots  (Fig.  236). 

The  dorsal  nerve-roots  have  a  very  different  origin.  In  em- 
bryos of  about  2.5  mm.,  in  which  the  medullary  canal  is  only 
partly  closed  (Fig.  54),  the  cells  which  lie  along  the  line  of  transi- 
tion between  the  lips  of  the  groove 
and  the  general  ectoderm  form  a 
distinct  ridge  readily  recognized 
in  sections  and  termed  the 
neural  crest  (Fig.  234,  A).  When 
the  lips  of  the  groove  fuse  to- 
gether the  cells  of  the  crest  unite 
to  form  a  wedge-shaped  mass, 
completing  the  closure  of  the 
canal  (Fig.  234,  B) ,  and  later  pro- 
liferate so  as  to  extend  outward 
over  the  surface  of  the  canal  (Fig. 
234,  C) .  Since  this  proliferation , 
is  most  active  in  the  regions  of 
the  crest  which  correspond  to 
the  mesodermic  somites  there  is 
formed  a  series  of  cell  masses,  ar- 
ranged segmentally  and  situated 
in  the  mesenchyme  at  the  sides  of  the  medullary  canal  (Fig.  219).. 
These  cell  masses  represent  the  dorsal  root  ganglia,  and  certain 
of  their  constituent  cells,  which  may  also  be  termed  neuroblasts, 
early  assume  a  fusiform  shape  and  send  out  a  process  from  each 
extremity.  One  of  these  processes,  the  axis-cylinder,  grows  in- 
ard  toward  the  medullary  canal  and  penetrates  its  marginal  velum, 
and,  after  a^longer  or  shorter  course  in  this  zone,  enters  the  mantle 
layer  and  comes  into  contact  with  the  dendrites  of  some  of  the 
central  neuroblasts.     The  other  process  extends  peripherally  and 


Fig.  234. — Three  Sections  through 
THE  Medullary  Canal  of  an  Embryo 
OF   2.5  MM. — (von  Lenhossek.) 


THE   HISTOGENESIS    OF   THE    NERVOUS    SYSTEM  385 

is  to  be  regarded  as  an  extremely  elongated  dendrite.  The  pro- 
cesses from  the  cells  of  each  ganglion  aggregate  to  form  a  nerve^ 
that  formed  by  the  axis-cylinders  being  the  posterior  root  of  a  spinal 
nerve,  while  that  formed  by  the  dendrites  soon  unites  with  the 
ventral  nerve-root  of  the  corresponding  segment  to  form  the  main 
stem  of  a  spinal  nerve. 

There  is  thus  a  very  important  difference  in  the  mode  of  de- 
velopment of  the  two  nerve-roots,  the  axis-cylinders  of  the 
ventral  roots  arising  from  cells  situated  in  the  wall  of  the  medul- 


FiG.  235. — Cells  from  the  Gasserian  Ganglion  of  a  Guinea-pig  Embryo. 
a,  Bipolar  cell;  b  and  c,  transitional  stages  to  d,  T-shaped  cells. — (van    Gehuchten.) 

lary  canal  and  growing  outward  (centrifugally)  while  those  of  the 
dorsal  root  spring  from  cells  situated  peripherally  and  grow  inward 
(centripe tally)  toward  the  medullary  canal.  In  the  majority 
of  the  dorsal  root  ganglia  the  points  of  origin  of  the  two  processes 
of  each  bipolar  cell  gradually  approach  one  another  (Fig.  235,  b) 
and  eventually  come  to  rise  from  a  common  stem,  a  process  of  the 
cell-body,  which  thus  assumes  a  characteristic  T  form  (Fig.  235,  (/). 

From  what  has  been  said  it  will  be  seen  that  each  axis-cylinder  is 
an'outgrowth  from  a  single  neuroblast  and  is  part  of  its  cell-body,  as  are 
also  the  dendrites.  Another  view  has,  however,  been  advanced  to  the 
effect  that  the  nerve  fibers  first  appear  as  chains  of  cells  and  that  the 
axis-cylinders,  being  differentiated  from  the  cytoplasm  of  the  chains, 
are  really  multicellular  products.  Many  difficulties  stand  in  the  way 
of  the  acceptance  of  this  view  and  recent  observations,  both  histo- 
genetic   (Cajal)   and  experimental   (Harrison),   tend   to  confirm   the 

25 


386  THE    SPINAL   CORD 

unicellular  origin  of  the  axis-cylinders.  The  embryological  evidence 
therefore  goes  to  support  the  neurone  theory^  which  regards  the  entire 
nervous  system  as  composed  of  definite  units,  each  of  which  corre- 
sponds to  a  single  cell  and  is  termed  a  neurone. 

By  the  development  of  the  axis-cylinders  which  occupy  the 
meshes  of  the  marginal  velum,  that  zone  increases  in  thickness  and 
comes  to  consist  principally  of  nerve-fibers,  while  the  cell-bodies  of 
the  neurones  of  the  cord  are  situated  in  the  mantle  zone.  No  such 
definite  distinction  of  color  in  the  two  zones  as  exists  in  the  adult 
is,  however,  noticeable  until  a  late  period  of  development,  the 
medullary  sheaths^  which  give  to  the  nerve-fibers  their  white  ap- 
pearance not  beginning  to  appear  until  the  fifth  month  and 
continuing  to  form  from  that  time  onward  until  after  birth.  The 
origin  of  the  myelin  which  composes  the  medullary  sheaths  is  as 
yet  uncertain,  although  the  more  recent  observations  tend  to  show 
that  it  is  picked  out  from  the  blood  and  deposited  around  the 
axis-cylinders  in  some  manner  not  yet  understood.  Its  appearance 
is  of  importance  as  being  associated  with  the  beginning  of  the 
definite  functional  activity  of  the  nerve-fibers. 

In  addition  to  the  medullary  sheaths  the  majortiy  of  the  fibers 
of  the  peripheral  nervous  system  are  provided  with  primitive 
sheaths,  which  are  lacking,  however,  to  the  fibers  of  the  central 
system.  They  are  formed  ty  cells  which  wander  out  from  the 
dorsal  root-ganglia  and  are  therefore  of  ectodermal  origin.  Frog 
larvae  deprived  of  their  neural  crests  at  an  early  stage  of  develop- 
ment produce  ventral  nerve-fibers  altogether  destitute  of  primi- 
tive sheaths  (Harrison) . 

Various  theories  have  been  advanced  to  account  for  the  formation  of 
the  medullary  sheaths.  It  has  been  held  that  the  myelin  is  formed 
at  the  expense  of  the  outermost  portions  of  the  axis-cylinders  them- 
selves (von  Kolliker),  and  on  the  other  hand,  it  has  been  regarded  as  an 
excretion  of  the  cells  which  compose  the  primitive  sheaths  surrounding 
the  fibers  (Ranvier),  a  theory  which  is,  however,  invalidated  by  the 
fact  that  myelin  is  formed  around  the  fibers  of  the  central  nervous 
system  which  possess  no  primitive  sheaths.  As  stated  above,  the 
more  recent  observations  (Wlassak)  indicate  its  exogenous  origin. 

It  has  been  seen  that  the  central  canal  is  closed  in  the  mid- 
dorsal  line  by  a  mass  of  cells  derived  from  the  neural  crest.     These 


THE    SPINAL   CORD  387 

cells  do  not  take  part  in  the  formation  of  the  mantle  layer,  but 
become  completely  converted  into  ependymal  tissue,  and  the^ 
same  is  true  of  the  cells  situated  in  the  mid-ventral  line  of  the  canal. 
In  these  two  regions,  known  as  the  roof -plate  and  floor -plate  re- 
spectively, the  wall  of  the  canal  has  a  characteristic  structure  and 
does  not  share  to  any  great  extent  in  the  increase  of  thickness 
which  distinguishes  the  other  regions  (Fig.  236).  In  the  lateral 
walls  of  the  canal  there  is  also  noticeable  a  differentiation  into  two 
regions,  a  dorsal  one  standing  in  relation  to  the  ingrowing  fibers 
from  the  dorsal  root  ganglia  and  known  as  the  dorsal  zone,  and  a 
ventral  one  the  ventral  zone,  similarly  related  to  the  ventral  nerve- 
roots.  In  different  regions  of  the  medullary  tube  these  zones,  as 
well  as  the  roof  and  floor-plates,  undergo  different  degrees  of  de- 
velopment, producing  peculiarities  which  may  now  be  considered. 
The  Development  of  the  Spinal  Cord.^Even  before  the  lips 
of  the  medullary  groove  have  met  a  marked  enlargment  of  the 
anterior  portion  of  the  canal  is  noticeable,  the  region  which  will 
become  the  brain  being  thus  distinguished  from  the  more  posterior 
portion  which  will  be  converted  into  the  spinal  cord.  When  the 
formation  of  the  mesodermic  somites  is  completed,  the  spinal  cord 
terminates  at  the  level  of  the  last  somite,  and  in  this  region  still 
retains  its  connection  with  the  ectoderm  of  the  dorsal  surface  of 
the  body;  but  in  that  portion  of  the  cord  which  is  posterior  to  the 
first  coccygeal  segment  the  histological  differentiation  does  not 
proceed  beyond  the  stage  when  the  walls  consist  of  several  layers 
of  similar  cells,  the  formation  of  neuroblasts  and  nerve-roots 
ceasing  with  the  segment  named.  After  the  fourth  month  the 
more  differentiated  portion  elongates  at  a  much  slower  rate  than 
the  surrounding  tissues  and  so  appears  to  recede  up  the  spinal 
canal,  until  its  termination  is  opposite  the  second  lumbar  vertebra. 
The  less  differentiated  portion,  which  retains  its  connection  with 
the  ectoderm  until  about  the  fifth  month,  is,  on  the  other  hand, 
drawn  out  into  a  slender  filament  whose  cells  degenerate  during 
the  sixth  month,  except  in  its  uppermost  part,  so  that  it  comes 
^  to  be  represented  throughout  the  greater  part  of  its  extent  by  a 
thin  cord  composed  of  pia  mater.     This  cord  is  the  structure 


388  THE    SPINAL   CORD 

known  in  the  adult  as  the  filum  terminale,  and  lies  in  the  center  of 
a  leash  of  nerves  occupying  the  lower  part  of  the  spinal  canal  and 
termed  the  cauda  equina.  The  existence  of  the  cauda  is  due  to 
the  recession  of  the  cord  which  necessitates  for  the  lower  Itimbar, 
sacral  and  coccygeal  nerves,  a  descent  through  the  spinal  canal  for 
a  greater  or  less  distance,  before  they  can  reach  the  intervertebral 
foramina  through  which  they  make  their  exit. 

In  the  early  stages  of  development  the  central  canal  of  the  cord 
is  quite  large  and  of  an  elongated  oval  form,  but  later  it  becomes 
somewhat  rhomboidal  in  shape  (Fig.  236,  A),  the  lateral  angles 
marking  the  boundaries  between  the  dorsal  and  ventral  zones. 
As  development  proceeds  the  sides  of  the  canal  in  the  dorsal  region 
gradually  approach  one  another  and  eventually  fuse,  so  that  this 
portion  of  the  canal  becomes  obliterated  (Fig.  136,  B)  and  is  indi- 
cated by  the  dorsal  longitudinal  fissure  in  the  adult  cord,  the 
central  canal  of  which  corresponds  to  the  ventral  portion  only  of 
the  embryonic  cavity.  While  this  process  has  been  going  on 
both  the  roof-  and  the  floor-plate  have  become  depressed  below 
the  level  of  the  general  surface  of  the  cord,  and  by  a  continuance  of 
the  depression  of  the  floor-plate — a  process  really  due  to  the  en- 
largement and  consequent  bulging  of  the  ventral  zone — the  an- 
terior median  fissure  is  produced,  the  difference  between  its  shape 
and  that  of  the  dorsal  fissure  being  due  to  the  difference  in  its 
development. 

The  development  of  the  mantle  layer  proceeds  at  first  more 
rapidly  in  the  ventral  zone  than  in  the  dorsal,  so  that  at  an  early 
stage  (Fig.  236,  A)  the  anterior  column  of  gray  matter  is  much 
more  pronounced,  but  on  the  development  of  the  dorsal  nerve- 
roots  the  formation  of  neuroblasts  in  the  dorsal  zone  proceeds  apace, 
resulting  in  the  formation  of  a  dorsal  column.  A  small  portion 
of  the  zone,  situated  between  the  point  of  entrance  of  the  dorsal 
nerve-roots  and  the  roof-plate,  fails,  however,  to  give  rise  to  neuro- 
blasts and  is  entirely  converted  into  ependyma.  This  represents 
the  iuture  funiculus  gracilis  (fasciculus  ofGoll)  (Fig.  236,  ^,cG)  and 
at  the  point  of  entrance  of  the  dorsal  roots  into  the  cord  a  well- 
marked  ovaLbundle  of  fibers  is  formed  (Fig.  236,  A,  ob)  which,  as 


THE    SPINAL   CORD 


389 


development  proceeds,  creeps  dorsally  over  the  surface  of  the  dorsal 
horn  until  it  meets  the  lateral  surface  of  the  funiculus  gracilis, 
and,  its  further  progress  toward  the  median  line  being  thus  impeded, 
it  insinuates  itself  between  that  fasciculus  and  the  posterior  horn 
to  form  the  funiculus  cuneatus  {fasciculus  of  Burdach)  (Fig.  236, 
B,cB). 


Fig.  236. — Transverse  Sections  through  the  Spinal  Cords  of  Embryos  of  (A) 
ABOUT  Four  and  a  Half  Weeks  and  (jB)  about  three  Months. 
cB,    funiculus  cuneatus;  cG,  funiculus  gracilis;  dh,  dorsal  column;  dz,  dorsal 
zone;  fp,  floor-plate;  oh,  oval  bundle;  rp,  roof-plate;  vh,   ventral  column;  vz,  ventral 
zone, — {His.) 

Little  definite  is  as  yet  known  concerning  the  development  of  the 
other  fasciculi  which  are  recognizable  in  the  adult  cord,  but  it  seems 
certain  that  the  lateral  and  anterior  cerebro-spinal  (pyramidal)  fasciculi 
are  composed  of  fibers  which  grow  downward  in  the  meshes  of  the 
marginal  velum  from  neuroblasts  situated  in  the  cerebral  cortex,  while 
the  cerebello-spinal  (direct  cerebellar)  fasciculi  and  the  fibers  of  the 
ground-bundles  have  their  origin  from  cells  of  the  mantle  layer  of  the 
cord. 

The  myelination  of  the  fibers  of  the  spinal  cord  begins  between  the 
fifth  and  sixth  months  and  appears  first  in  the  funiculi  cuneati,  and  about 
a  month  later  in  the  funiculi  graciles.  The  myelination  of  the  great 
motor  paths,  the  lateral  and  anterior  cerebro-spinal  fasciculi,  is  the  last 
to  develop,  appearing  toward  the  end  of  the  ninth  month  of  fetal  life. 


390 


THE   BRAIN 


my 


The  Development  of  the  Brain. — The  enlargement  of  the 
anterior  portion  of  the  medullary  canal  does  not  take  place  quite 
uniformly,  but  is  less  along  two  transverse  lines  than  elsewhere,  so 
that  the  brain  region  early  becomes  divided  into  three  primary 
vesicles  which  undergo  further  differentiation  as  follows.  Upon 
each  side  of  the  anterior  vesicle  an  evagination  appears  and  be- 
comes converted  into  a  club-shaped 
structure  attached  to  the  ventral 
portion  of  the  vesicle  by  a  pedicle. 
These  evaginations  (Fig.  237,  op) 
are  known  as  the  optic  evaginations, 
and  being  concerned  in  the  formation 
of  the  eye  will  be  considered  in  the 
succeeding  chapter.  After  their  for- 
mation the  antero-lateral  portions  of 
the  vesicle  become  bulged  out  into 
two  proturberances  (h)  which  rapidly 
increase  in  size  and  give  rise  eventu- 
ally to  the  two  cerebral  hemispheres, 
which  form,  together  with  the  por- 
tion of  the  vesicle  which  lies  between 
them,  what  is  termed  the  telence- 
phalon or  fore-brain,  the  remainder  of 
the  vesicle  giving  rise  to  what  is  known 
as  the  diencephalon  or  Hween-brain 
(Fig.  237,  t).  The  middle  vesicle  is 
bodily  converted  into  the  mesence- 
phalon or  mid-brain  (m),  but  the 
posterior  vesicle  differentiates  so  that 
threepartsmay  be  recognized:  (i)  a 
rather  narrow  portion  which  immediately  succeeds  the  mid-brain 
and  is  termed  the  isthmus  (i) ;  (2)  a  portion  whose  roof  and  floor 
give  rise  to  the  cerebellum  and  pons  respectively,  and  which  is 
termed  the  metencephalon  or  hindbrain  (mt) ;  and  (3)  a  terminal 
portion  which  is  known  as  the  medulla  oblongata,  or,  to  retain  a 
consistent  nomenclature,  the  myelencephalon  or  after-brain  {my). 


Fig.  237. — Reconstruction  of 
THE  Brain  of  an  Embryo  of  2.15 

MM. 

h^  Hemisphere;  i,  isthmus;  m, 
mesencephalon;  mf,  mid-brain  flex- 
ure; mt,  metencephalon;  my,  my- 
elencephalon; nf,  nape  flexure;  ol, 
otic  capstile;  op,  optic  evagina- 
tion; /,  diencephalon. — {His.) 


THE   BRAIN 


39i 


From    each  of  these  six  divisions  definite  structures  arise  whose 
relations  to  the  secondary  divisions  and  to  the  primary  vesicles 
may  be  understood  from  the  following  table  and  from  the  annexed" 
figure  (Fig.  238),  which  represents  a  median  longitudinal  section 
of  the  brain  of  a  fetus  of  three  months. 


3d  Vesicle. 


Myelencephalon 
Metencephalon 


2nd  Vesicle. 


ist  Vesicle. 


Isthmus 


Mesencephalon 


Diencephalon 


Telencephalon 


Medulla  oblongata  (i). 

/  Pons  (II  i). 

\  Cerebellum  (II  2). 

(Brachia  conjunctiva. 
Cerebral  peduncles  (posterior  por- 
tion) (III). 

{Cerebral  peduncles  (anterior  por- 
tion) (IV  i). 
Corpora  quadrigemina  (IV  2). 

{Pars  mammillaris  (V  i). 
Thalamus  (V  2). 
Epiphysis  (V  3). 

Infundibulum  (VI  i). 
Corpus  striatum  (VI  2). 
Olfactory  bulb  (VI  3). 
Hemispheres  (VI  4). 


But  while  the  walls  of  the  primary  vesicles  undergo  this  com 
plex  differentiation,   their  cavities  retain  much  more  perfectly 
their  original   relations,   only  that  of   the  first  sharing   to  any 
great  extent  the  modifications  of  the  walls. 

The  cavity  of  the  third  vesicle  persists  in  the  adult  as  the  fourth 
ventricle,  traversing  all  the  subdivisions  of  the  vesicle;  that  of  the 
second,  increasing  but  little  in  height  and  breadth,  constitutes  the 
aqu(Bd7ictus  cerebri  {iter) ;  while  that  of  the  first  vesicle  is  continued 
into  the  cerebral  hemispheres  to  form  the  lateral  ventricles,  the  re- 
mainder of  it  constituting  the  third  ventricle,  which  includes  the 
cavity  of  the  median  portion  of  the  telencephalon  as  well  as  the 
entire  cavity  of  the  diencephalon. 

During  the  differentiation  of  the  various  divisions  of  the  brain 
certain  flexures  appear  in  the  roof  and  floor,  and  to  a  certain  ex- 


392  THE   BRAIN 

tent  correspond  with  those  already  described  as  occurring  in  the 
embryo.  The  first  of  these  flexures  to  appear  occurs  in  the  region 
of  the  mid-brain,  the  first  vesicle  being  bent  ventrally  until  it 
comes  to  lie  at  practically  a  right  angle  with  the  axis  of  the  mid- 
brain. This  may  be  termed  the  mid-brain  flexure  (Fig.  237,  mf) 
and  corresponds  with  the  head-bend  of  the  embryo.  The  second 
flexure  occurs  in  the  region  of  the  medulla  oblongata  and  is  known 
as  the  nape  flexure  (Fig.  237,  nf)\  it  corresponds  with  the  similarly 
named  bend  of  the  embryo  and  is  produced  by  a  bending  ventrally 
of  the  entire  head,  so  that  the  axis  of  the  mid-brain  comes  to  lie 


]\Z 


Fig.  238. — Median  Longitudinal  Section  of  the  Brain  of  an  Embryo  of  tiiic 
Third  Month. — {His.) 

almost  at  right  angles  with  that  of  the  medulla  and  that  of  the 
first  vesicle  parallel  with  it.  Finally,  a  third  flexure  occurs  in 
the  region  of  the  metencephalon  and  is  entirely  peculiar  to  the 
nervous  system;  it  consists  of  a  bending  ventrally  of  the  floor  of 
the  hind-brain,  the  roof  of  this  portion  of  the  brain  not  being 
affected  by  it,  and  it  may  consequently  be  known  as  the  pons 
flexure  (Fig.  238). 

In  the  later  development  the  pons  flexure  practically  dis- 
appears, owing  to  the  development  in  this  region  of  the  transverse 
fibers  and  nuclei  of  the  pons,  but  the  mid-brain  and  nape  flexures 


THE   MYELENCEPHALON  393 

persist,  though  greatly  reduced  in  acuteness,  the  axis  of  the 
anterior  portion  of  the  adult  brain  being  inclined  to  that  of  the^ 
medulla  at  an  angle  of  about  134  degrees. 

The  Development  of  the  Myelencephalon. — In  its  posterior 
portion  the  myelencephalon  closely  resembles  the  spinal  cord  and 
has  a  very  similar  development.  More  anteriorly,  however,  the 
roof -plate  (Fig.  239,  rp)  widens  to  form  an  exceedingly  thin  mem- 
brane, the  posterior  velum;  with  the  broadening  of  the  roof -plate 
there  is  associated  a  broadening  of  the  dorsal  portion  of  the  brain 
cavity,  the  dorsal  and  ventral  zones  bending  outward,  until,  in 
the  anterior  portion  of  the  after-brain,  the  margins  of  the  dorsal 
zone  have  a  lateral  position,  and  are,  indeed,  bent  ventrally  to 
form  a  reflected  lip  (Fig.  239,  /).  The  portion  of  the  fourth  ven- 
tricle contained  in  this  division  of  the  brain  becomes  thus  con- 
verted into  a  broad  shallow  cavity,  whose  floor  is  formed  by  the 
ventral  zones  separated  in  the  median  line  by  a  deep  groove,  the 
floor  of  which  is  the  somewhat  thickened  floor-plate.  About  the 
fourth  month  there  appears  in  the  roof-plate  a  transverse  groove 
into  which  the  surrounding  mesenchyme  dips,  and,  as  the  groove 
deepens  in  later  stages,  the  mesenchyme  contained  within  it  be- 
comes converted  into  blood-vessels,  forming  the  chorioid  plexus 
of  the  fourth  ventricle,  a  structure  which,  as  may  be  seen  from 
its  development,  does  not  lie  within  the  cavity  of  the  ventricle, 
but  is  separated  from  it  by  the  portion  of  the  roof-plate  which 
forms  the  floor  of  the  groove. 

In  embryos  of  about  9  mm.  the  differentiation  of  the  dorsal 
and  ventral  zones  into  ependymal  and  mantle  layers  is  clearly 
visible  (Fig.  239),  and  in  the  ventral  zone  the  marginal  velum  is 
also  well  developed.  Where  the  fibers  from  the  sensory  ganglion 
of  the  vagus  nerve  enter  the  dorsal  zone  an  oval  area  (Fig.  239, 
fs)  is  to  be  seen  which  is  evidently  comparable  to  the  oval  bundle 
of  the  cord  and  consequently  with  the  fasciculus  of  Burdach.  It 
gives  rise  to  the  solitary  fasciculus  of  adult  anatomy,  and  in 
embryos  of  ii  to  13  mm.  it  becomes  covered  in  by  the  fusion  of 
the  reflected  lip  of  the  dorsal  zone  with  the  sides  of  the  myelen- 
cephalon, this  fusion,  at  the  same  time,  drawing  the  margins  of 


394  THE    MYELENCEPHALON 

the  roof-plate  ventrally  to  form  a  secondary  lip  (Fig.  240) .  Soon 
after  this  a  remarkable  migration  ventrally  of  neuroblasts  of  the 
dorsal  zone  begins.  Increasing  rapidly  in  number  the  migrating 
cells  pass  on  either  side  of  the  solitary  fasciculus  toward  the 
territory  of  the  ventral  zone,  and,  passing  ventrally  to  the  ventral 
portion  of  the  mantle  layer,  into  which  fibers  have  penetrated 
and  which  becomes  the  formatio  reticularis  (Fig.  240,  /r),  they 
differentiate  to  form  the  olivary  body  (ol). 


Fig.  239. — Transverse    Section    through    the    Medulla    Oblongata    of    an 

Embryo  of  9.1  mm. 

dz,   Dorsal  zone;  fp,  floor-plate;  fs,  fasciculus  solitarius;  I,  lip;  rp,  roof-plate;    vz, 

ventral  zone;  X  and  XII,  tenth  and  twelfth  nerves. — (His.) 

The  thickening  of  the  floor-plate  gives  opportunity  for  fibers 
to  pass  across  the  median  line  from  one  side  to  the  other,  and  this 
opportunity  is  taken  advantage  of  at  an  early  stage  by  the  axis- 
cylinders  of  the  neuroblasts  of  the  ventral  zone,  and  later,  on  the 
establishment  of  the  olivary  bodies,  other  fibers,  descending  from 
the  cerebellum,  decussate  in  this  region  to  pass  to  the  olivary  body 
of  the  opposite  side.  In  the  lower  part  of  the  medulla  fibers  from 
the  neuroblasts  of  the  nuclei  gracilis  and  cuneatus,  which  seem 
to  be  developments  from  the  mantle  layer  of  the  dorsal  zone^  also 
decussate  in  the  substance  of  the  floor-plate;  these  fibers,  known 
as  the  arcuate  fibers,  pass  in  part  to  the  cerebellum,  associating 
themselves  with  fibers  ascending  from  the  spinal  cord  and  with 
the  olivary  fibers  to  form  a  round  bundle  situated  in  the  dorsal 


THE    METENCEPHALON   AND    ISTHMUS  395 

portion  of  the  marginal  velum  and  known  as  the  restiform  lody 
(Fig.  240,  tr). 

The  principal   differentiations  of   the  zones  of   the  myelen- 
cephalon  may  be  stated  in  tabular  form  as  follows: 

Roof -plate Posterior  velum. 

(Nuclei  of  termination  of  sensory  roots  of  cranial  nerves. 
Nuclei  gracilis  and  cuneatus. 
The  olivary  bodies. 

,^         ,  J  Nuclei  of  origin  of  the  motor  roots  of  cranial  nerves. 

Ventral  zones <  ^,        ^.    ,      , 

I  The  reticular  formation. 

Floor-plate The  median  raphe. 


Fig.  240. — Transverse  Section  through  the  Medulla  Oblongata  of  an  Em- 
bryo OF  about  Eight  Weeks. 
av.  Ascending  root  of  the  trigeminus;  fr,  reticular  formation;  ol,  olivary  body;    sf, 
solitary  fasciculus;  tr,  restiform  body;  XII,  hypoglossal  nerve. — (His.) 


The  Development  of  the  Metencephalon  and  Isthmus. — Our 
knowledge  of  the  development  of  the  metencephalon,  isthmus,  and 
mesencephalon  is  by  no  means  as  complete  as  is  that  of  the 
myelencephalon.  The  pons  develops  as  a  thickening  of  the  por- 
tion of  the  brain  floor  which  forms  the  anterior  wall  of  the  pons 
flexure,  and  its  transverse  fibers  are  well  developed  by  the  fourth 
month  (Mihalkovicz),  but  all  details  regarding  the  origin  of  the 
pons  nuclei  are  as  yet  wanting.  If  one  may  argue  from  what  oc- 
curs in  the  myelencephalon,  it  seems  probable  that  the  reticular 
formation  of  the  metencephalon  is  derived  from  the  ventral  zone, 
and  that  the  median  raphe  represents  the  floor-plate.     Further- 


396 


THE    CEREBELLUM 


more,  the  relations  of  the  pons  nuclei  to  the  reticular  formation  on 
the  one  hand,  and  its  connection  by  means  of  the  transverse  pons 
fibers  with  the  cerebellum  on  the  other,  suggest  the  possibility 
that  they  may  be  the  metencephalic  representatives  of  the 
olivary  bodies  and  are  formed  by  a  migration  ventrally  of  neuro- 
blasts from  the  dorsal  zones,  such  a  migration  having  been  ob- 
served to  occur  (Essick). 

The  cerebellum  is  formed  from  the  dorsal  zones  and  roof-plate 
of  the  metencephalon  and  is  a  thickening  of  the  tissue  imme- 
diately anterior  to  the  front  edge  of  the  posterior  velum.  This 
latter  structure  has  in  early  stages  a  rhomboidal  shape  (Fig.  241, 


Fig.  241. — A,  Dorsal  View  of  the  Brain  of  a  Rabbit  Embryo  of  16  mm.; 
Median  Longitudinal  Section  of  a  Calf  Embryo  of  3  cm. 

c,  Cerebellum;  m,  mid-brain. — (Mihalkovicz.) 

A)  which  causes  the  cerebellar  thickening  to  appear  at  first  as  if 
composed  of  two  lateral  portions  inclined  obliquely  toward  one 
another.  In  reality,  however,  the  thickening  extends  entirely 
across  the  roof  of  the  brain  (Fig.  241,  B),  the  roof-plate  probably 
being  invaded  by  cells  from  the  dorsal  zones  and  so  giving  rise  to 
the  vermis,  while  the  lobes  are  formed  directly  from  the  dorsal 
zones.  During  the  second  month  a  groove  appears  on  the  ventral 
surface  of  each  lobe,  marking  out  an  area  which  becomes  the  floc- 
culus, and  later,  during  the  third  month,  transverse  furrows  appear 
upon  the  vermis  dividing  it  into  five  lobes,  and  later  still  extend 
out  upon  the  lobes  and  increase  in  number  to  produce  the  lamellate 
structure  characteristic  of  the  cerebellum. 

The  histogenetic  development  of  the  cerebellum  at  first  pro- 
ceeds along  the  lines  which  have  already  been  described  as  typical, 


THE    CEREBELLUM 


397 


but  after  the  development  of  the  mantle  layer  the  cells  lining  the 
greater  portion  of  the  cavity  of  the  ventricle  cease  to  multiply, 
only  those  which  are  situated  in  the  roof-plate  of  the  metencepha- 
lon  and  along  the  line  of  junction  of  the  cerebellar  thickening 
with  the  roof-plate  continuing  to  divide.  The  indifferent  cells 
formed  in  these  regions  migrate  outward  from  the  median  line 
and  forward  in  the  marginal  velum  to  form  a  superficial  layer, 
known  as  the  epithelioid  layer ^  and  cover  the  entire  surface  of 
cerebellum.  (Fig.  242).  The 
cells  of  this  layer,  like  those  of 
the  mantle,  differentiate  into 
neuroglia  cells  and  neuro- 
blasts, the  latter  for  the  most 
part  migrating  centrally  at  a 
later  stage  to  mingle  with  the 
cells  of  the  mantle  layer  and 
to  become  transformed  into 
the  granular  cells  of  the  cere- 
bellar cortex.  The  neuroglia 
cells  remain  at  the  surface, 
however,  forming  the  prin- 
cipal constituent  of  the  outer 
or,  as  it  is  now  termed,  the 
molecular  layer  of  the  cortex, 
and  into  this  the  dendrites  of 
the  Purkinje  cells,  probably  derived  from  the  mantle  layer,  pro- 
ject. The  migration  of  the  neuroblasts  of  the  epithelial  layer  is 
probably  completed  before  birth,  at  which  time  but  few  remain  in 
the  molecular  layer  to  form  the  stellate  cells  of  the  adult.  The 
origin  of  the  dentate  and  other  nuclei  of  the  cerebellum  is  at 
present  unknown,  but  it  seems  probable  that  they  arise  from 
cells  of  the  mantle  layer. 

The  nerve-fibers  which  form  the  medullary  substance  of  the 
cerebellum  do  not  make  their  appearance  until  about  the  sixth 
month,  when  they  are  to  be  found  in  the  ependymal  tissue  on 
the  inner  surface  of  the  layer  of  granular  cells.     Those  which 


Fig.  242. — Diagram  Representing  the 
Differentiation  of  the  Cerebei^lar 
Cells, 

The  circles,  indifferent  cells;  circles  -with 
dots,  neuroglia  cells;  shaded  cells,  germinal 
cells;  circles  with  cross,  germinal  cells  in 
mitosis;  black  cells,  nerve-cells.  L,  Lateral 
recess;  M,  median  furrow,  and  R,  floor  of 
IV,  fourth  ventricle. — {Schapef.) 


398  THE   ISTHMUS 

are  not  commissural  or  associative  in  function  converge  to  the 
line  of  junction  of  the  cerebellum  with  the  pons,  and  there  pass 
into  the  marginal  velum  of  the  pons,  myelencephalon,  or  isthmus 
as  the  case  may  be. 

The  dorsal  surface  of  the  isthmus  is  at  first  barely  distinguish- 
able from  the  cerebellum,  but  as  development  proceeds  its  roof- 
plate  undergoes  changes  similar  to  those  occurring  in  the  medulla 
oblongata  and  becomes  converted  into  the  anterior  velum.  In 
the  dorsal  portion  of  its  marginal  velum  fibers  passing  to  and  from 
the  cerebellum  appear  and  form  the  hrachia  conjunctiva,  while 
ventrally  fibers,  descending  from  the  more  anterior  portions  of  the 
brain,  form  the  cerebral  peduncles.  Noting  is  at  present  known 
as  to  the  history  of  the  gray  matter  of  this  division  of  the  brain, 
although  it  may  be  presumed  that  its  ventral  zones  take  part  in 
the  formation  of  the  tegmentum,  while  from  its  dorsal  zones  the 
nuclei  of  the  brachia  conjunctiva  are  possibly  derived. 

The  following  table  gives  the  origin  of  the  principal  structures 
of  the  metencephalon  and  isthmus : 

Metencephalon  Isthmus 

Roof-plate.  > /  Posterior  velum.  Anterior  velum. 

\  Vermis  of  cerebellum. 

f  Lobes  of  cerebellum.  Brachia  conjunctiva. 

iFlocculi. 
Nuclei  of  termination  of  sen- 
sory roots  of  cranial  nerves. 
,^  [  Pons  nuclei. 

Nuclei  of  origin  of   motor  Posterior  part  of  cere- 
roots  of  cranial  nerves.  bral  peduncles. 

Ventral  zones 1  Reticular  formation.  Posterior  part  of  teg- 

[  mentum. 

Floor-plate Median  raphe.  Median  raphe. 

The  Development  of  the  Mesencephalon. — Our  knowledge  of  the 
development  of  this  portion  of  the  brain  is  again  very  imperfect. 
During  the  stages  when  the  flexures  of  the  brain  are  well  marked 
(Figs.  237  and  238)  it  forms  a  very  prominent  structure  and  pos- 
sesses for  a  time  a  capacious  cavity.    Later,  however,  it  increases 


THE    MESENCEPHALON  399 

in  size  less  rapidly  than  adjacent  parts  and  its  wall  thicken,  the 
roof-  and  floor-plates  as  well  as  the  zones,  and,  as  a  result,  the 
cavity  becomes  the  relatively  smaller  canal-like  cerebral  aquaeduct. 
In  the  marginal  velum  of  its  ventral  zone  fibers  appear  at  about 
the  third  month,  forming  the  anterior  portion  of  the  cerebral 
peduncles,  and,  at  the  same  time,  a  median  longitudinal  furrow 
appears  upon  the  dorsal  surface,  dividing  it  into  two  lateral  eleva- 
tions which,  in  the  fifth  month,  are  divided  transversely  by  a 
second  furrow  and  are  thus  converted  from  corpora  bigemina  (in 
which  form  they  are  found  in  the  lower  vertebrates)  into  corpora 
quadrigemina. 

Nothing  is  known  as  to  the  differentiation  of  the  gray  matter  of  the 
dorsal  and  ventral  zones  of  the  mid-brain.  From  the  relation  of  the 
parts  in  the  adult  it  seems  probable  that  in  addition  to  the  nuclei  of  ori- 
gin of  the  oculomotor  and  trochlear  nerves,  the  ventral  zones  give  origin 
to  the  gray  matter  of  the  tegmentum,  which  is  the  forward  continuation 
of  the  reticular  formation.  Similarly  it  may  be  supposed  that  the  cor- 
pora quadrigemina  are  developments  of  the  dorsal  zones,  as  may  also  be 
the  red  nuclei,  whose  relations  to  the  brachia  conjunctiva  suggest  a  com- 
parison with  the  olivary  bodies  and  the  nuclei  of  the  pons. 

A  tentative  scheme  representing  the  origin  of  the  mid-brain  structures 
may  be  stated  thus: 

Roof-plate (?) 

Dorsal  zones f  Corpora  quadrigemina. 

\  Red  nuclei. 

Ventral  zones J  Anterior  part  of  tegmentum. 

\  Anterior  part  of  cerebral  peduncles. 
Floor-plate Median  raphe. 

The  Development  of  the  Diencephalon.—A  transverse  section 
through  the  diencephalon  of  an  embryo  of  about  five  weeks  (Fig. 
243)  shows  clearly  the  differentiation  of  this  portion  of  the  brain 
into  the  typical  zones,  the  roof-plate  (rp)  being  represented  by  a 
thin-walled,  somewhat  folded  area,  the  floor-plate  (fp)  by  the 
tissue  forming  the  floor  of  a  well-marked  ventral  groove,  while 
each  lateral  wall  is  divided  into  a  dorsal  and  ventral  zone  by  a 
groove  known  as  the  sulcus  Monroi  (Sm),  which  extends  forward 
and  ventrally  toward  the  point  of  origin  of  the  optic  evagination 


400 


THE   DTENCEPHALON 


(Fig.  245).  At  the  posterior  end  of  the  ridge-like  elevation  which 
represents  the  roof-plate  is  a  rounded  elevation  (Fig.  244,  p)  which, 
in  later  stages,  elongates  until  it  almost  reaches  the  dermis,  form- 
ing a  hollow  evagination  of  the  brain  roof  known  as  the  pineal 
process.  The  distal  extremity  of  this  process  enlarges  to  a  sac- 
like structure  which  later  becomes  lobed,  and,  by  an  active 
proliferation  of  the  cells  lining  the  cavities  of  the  various  lobes, 
finally  becomes  a  solid  structure,   the  pineal  body.     The   more 

proximal  portion  of  the  evagination, 
'"'*  remaining   hollow,    forms   the  pineal 

stalky  and  the  entire  structure,  body 

and  stalk,  constitutes  what  is  known 

as  the  epiphysis. 


Fig.  243. — Transverse  Sec- 
tion   OF    THE   DiENCEPHALON   OF 

AN  Embryo  of  Five  Weeks. 

dz,  dorsal  zone;//?,  floor-plate; 
rp,  roof-plate;  Sm,  sulcus  Monroi; 
vz,  ventral  zone. — {His.) 


The  significance  of  this  organ  in  the 
Mammalia  is  doubtful.  In  the  Reptilia 
and  other  lower  forms  the  outgrowth  is 
double,  a  secondary  outgrowth  arising 
from  the  base  or  from  the  anterior  wall 
of  the  primary  one.  This  anterior  evagi- 
nation elongates  until  it  reaches  the 
dorsal  epidermis  of  the  head,  and,  there 
expanding,  develops  into  an  unpaired 
eye,  the  epidermis  which  overlies  it  be- 
coming converted  into  a  transparent 
cornea.  In  the  Mammaha  this  anterior 
process  does  not  develop  and  the  epiphy- 
sis in  these  forms  is  comparable  only  to  the  posterior  process  of  the 
Reptilia. 

In  addition  to  the  epiphysial  evaginations,  another  evagination  arises 
from  the  roof -plate  of  the  first  brain  vesicle,  further  forward,  in  the  region 
which  becomes  the  median  portion  of  the  telencephalon.  This  para- 
physis  as  it  has  been  called,  has  been  observed  in  the  lower  vertebrates 
and  in  the  Marsupials  (^Selenka),  but  up  to  the  present  has  not  been 
found  in  other  groups  of  the  Mammalia.  It  seems  to  be  comparable 
to  a  chorioid  plexus  which  is  e  vagina  ted  from  the  brain  surface  instead  of 
being  invaginated  as  is  usually  the  case.  There  is  no  evidence  that  a 
paraphysis  is  developed  in  the  human  brain. 

The  portion  of  the  roof -plate  which  lies  in  front  of  the  epiphysis 
represents  the  velum  interpositum  of  the  adult  brain,  and  it  forms 
at  first  a  distinct  ridge  (Fig.  244,  rp).     At  an  early  stage,  however, 


THE   DIENCEPHALON 


401. 


it  becomes  reduced  to  a  thin  membrane  upon  the  surface  of  which 
blood-vessels,  developing  in  the  surrounding  mesenchyme,  arrange 
themselves  at  about  the  third  month  in  two  longitudinal  plexuses, 
which,  with  the  subjacent  portions  of  the  velum,  become  in- 
vaginated  into  the  cavity  of  the 
third  ventricle  to  form  its  chorioid 
plexus. 

The  dorsal  zones  thicken  in 
their  more  dorsal  and  anterior 
portions  to  form  massive  struc- 
tures, the  thalami  (Figs.  237,  V2, 
and  244,  ot),  which,  encroaching 
upon  the  cavity  of  the  ventricle, 
transform  it  into  a  narrow  slit- 
like space,  so  narrow,  indeed,  that 
at  about  the  fifth  month  the  inner 
surfaces  of  the  two  thalami  come 
in  contact  in  the  median  line, 
forming  what  is  known  as  the 
intermediate  mass.  More  ven- 
trally  and  posteriorly  another 
thickening  of  the  dorsal  zone 
occurs,  giving  rise  on  each  side 
to  the  pulvinar  of  the  thalamus 
and  to  a  lateral  geniculate  body, 
and  two  ridges  extending  back- 
ward and  dorsally  from  the  latter 
structures  to  the  thickenings  in 
the  roof  of  the  mid-brain  which 
represent  the  anterior  corpora 
quadrigemina,  give  a  path  along 
which  the  nerve-iibers  which  constitute  the  superior  quadri- 
geminal  brachia  pass. 

From  the  ventral  zones  what  is  known  as  the  hypothalamic 
region  develops,  a  mass  of  fibers  and  cells  whose  relations  and 
development  are  not  yet  clearly  understood,  but  which  may  be 

26 


Fig.  224. — Dorsal  View  of  the 
Brain,  the  Roof  of  the  Lateral 
Ventricles  being  Removed,  of  an 
Embryo  of  1.36  mm. 

b,  Superior  brachium;  eg,  lateral 
geniculate  body;  c/»,  choroid  plexus;  cqa, 
anterior  corpus  quadrigeminun^;  h,  hip- 
pocampus; hf,  hippocampal  fissure;  ot, 
thalamus;  p,  pineal  body;  rp,  roof-plate. 
—(His.) 


402  THE    TELENCEPHALON 

regarded  as  the  forward  continuation  of  the  tegmentum  and 
reticular  formation.  In  the  median  line  of  the  floor  of  the  ventricle 
an  unpaired  thickening  appears,  representing  the  corpora  mammil- 
laria,  which  during  the  third  month  becomes  divided  by  a  median 
furrow  into  two  rounded  eminences;  but  whether  these  structures 
and  the  posterior  portion  of  the  tuber  cinereum,  which  also  develops 
from  this  region  of  the  brain,  are  derivatives  of  the  ventral  zones  or 
of  the  floor-plate  is  as  yet  uncertain. 

Assuming  that  the  mammillaria  and  the  tuber  cinereum  are  de- 
rived from  the  ventral  zones,  the  origins  of  the  structures  formed 
from  the  walls  of  the  diencephalon  may  be  tabulated  as  follows: 

Roof-plate f  Velum  interpositum. 

[  Epiphysis. 

f  Thalami. 
Dorsal  zones I  Pulvinares. 

1  Lateral  geniculate  bodies. 

I  Hypothalamic  region. 
Ventral  zones ^  Corpora  mammillaria. 

[  Tuber  cinereum  (in  part). 
Floor-plate Tissue  of  mid- ventral  line. 

The  Development  of  the  Telencephalon. — For  convenience  of 
description  the  telencephalon  may  be  regarded  as  consisting  of  a 
median  portion,  which  contains  the  anterior  part  of  the  third  ven- 
tricle, and  two  lateral  outgrowths  which  constitute  the  cerebral 
hemispheres.  The  roof  of  the  median  portion  undergoes  the  same 
transformation  as  does  the  greater  portion  of  that  of  the  dienceph- 
alon and  is  converted  into  the  anterior  part  of  the  velum  inter- 
positum (Fig.  246,  vi).  Anteriorly  this  passes  into  the  anterior 
wall  of  the  third  ventricle,  the  lamina  terminalis  (It),  a  structure 
which  is  to  be  regarded  as  formed  by  the  union  of  the  dorsal  zones 
of  opposite  sides,  since  it  lies  entirely  dorsal  to  the  anterior  end 
of  the  sulcus  Monroi.  From  the  ventral  part  of  the  dorsal  zones 
the  optic  evaginations  are  formed,  a  depression,  the  optic  recess 
(or),  marking  their  point  of  origin. 

The  ventral  zones  are  but  feebly  developed,  and  form  the 
anterior  part  of  the  hypothalamic  region,  while  at  the  anterior 
extremity  of  the  floor-plate  an  evagination  occurs,  the  infundibular 


THE    TELENCEPHALON  403 

recess  {ir),  which  elongates  to  form  a  funnel-shaped  structure 
known  as  the  hypophysis.  At  its  extremity  the  hypophysis  comes 
in  contact  during  the  fifth  week  with  the  enlarged  extremity  of 
Rathke's  pouch  formed  by  an  invagination  of  the  roof  of  the  oral 
sinus  (see  p.  287),  and  applies  itself  closely  to  the  posterior  surface 
of  this  (Fig.  238),  to  form  with  it  the  pituitary  body.  The  anterior 
lobe  at  an  early  stage  separates  from  the  mucous  membrane  of 
the  oral  sinus,  the  stalk  by  which  it  was  attached  completely  dis- 
appearing, and  the  posterior  wall  of  the  sack  so  formed  wraps  itself 


Pig.  245. — Diagram  Showing  the  Relationships  of  the  Pituitary  Body  in 
THE  Adult  Brain.     (Tilney.) 
cm,    corpus    mammillare;    inf,   inf undibulum ;    It,   lamina   terminalis;   oc,    optic 
commissure.     The  infundibulum  portion  of  the  pituitary  body  is  represented  in 
solid  black,  the  tuberal  portion  is  stippled  and  the  anterior  lobe  cross  hatched. 

around  the  downgrowth  from  the  brain,  completely  investing 
it  and  forming  the  infundibular  portion  (Fig.  245)  of  the  pituitary 
body  (Tilney).  From  the  ventral  portion  of  the  anterior  wall 
an  outgrowth  appears  on  either  side,  and  these,  growing  upwards 
aver  the  lateral  surfaces  of  the  sack,  push  their  way  between  it  and 
the  base  of  the  diencephalon,  uniting  in  the  middle  line  and  also 
sending  a  process  backwards  to  surround  the  upper  part  of  the 


404  THE    TELENCEPHALON 

stalk  (infundibulum)  connecting  the  hypophysis  with  the  brain- 
The  plate  of  pituitary  tissue  thus  formed,  resting  upon  the 
under  surface  of  that  portion  of  the  floor  of  the  diencephalon 
which  becomes  the  tuber  cinereum,  gives  rise  to  what  is 
termed  the  tuheral  portion  of  the  pituitary  body,  (Fig.  245) 
while  the  anterior  wall  of  the  oval  sack,  sending  out  proces- 
ses into  the  surrounding  mesenchyme  and  so  giving  rise  to  a  mass 
of  solid  cords  of  cells  embedded  in  a  mesenchyme  rich  in  blood 
vessels,  forms  what  is  termed  the  anterior  lobe  or  distal  portion. 
The  tuberal  portion  becomes  also  highly  vascular  and  its  cells 
arrange  themselves  in  cords,  which,  however  possess  a  lumen 
in  which  a  colloid  secretion  collects  in  later  stages;  the  infundi- 
bular portion  does  not  become  vascularized  to  any  great  extent 
and  its  cells  form  a  many-layered  epithelial  investment  of  the 
hypophysis,  while  between  it  and  the  distal  portion  a  cleftlike 
space  remains  as  the  representative  of  the  cavity  of  the  original 
oral  sack.  The  cavity  of  the  hypophysis,  which  in  early  stages 
is  quite  large,  later  becomes  obliterated  except  in  its  proximal 
portion  and  the  neural  downgrowth  thus  becomes  converted 
into  a  solid  mass  composed  of  neuroglia  cells  and  fibers,  among 
which  some  colloid  material  may  occasionally  be  present. 

The  cerebral  hemispheres  are  formed  from  the  lateral  portions 
of  the  dorsal  zones,  each  possessing  also  a  prolongation  of  the 
roof-plate.  From  the  more  ventral  portion  of  each  dorsal  zone 
there  is  formed  a  thickening,  the  corpus  striatum  (Figs.  246,  cs^ 
and  238,  F/  2) ,  a  structure  which  is  for  the  telencephalon  what  the 
optic  thalamus  is  for  the  diencephalon.  It  is  at  first  anterior  to 
and  quite  separate  from  the  thalamus  (Fig.  243)  but  later,  as  it 
enlarges,  it  extends  backwards  so  as  to  overlie  the  anterior  part 
of  the  thalamus  laterally  and  fuses  with  it  so  that  the  two  gang- 
lionic masses  become  continuous,  though  the  area  of  contact 
remains  indicated  by  the  tcsnia  semicircularis.  When  the  pro- 
jection fibers  from  the  cerebral  cortex  develop  they  converge  to 
form  a  definite  band,  the  internal  capsule,  the  posterior  portion 
of  which  passes  between  the  thalamus  and  the  corpus  striatum, 
while   the  anterior  portion  passes  through  the  substance  of  the 


THE    TELENCEPHALON  405 

striatum,  dividing  it  into  two  portions,  the  nucleus  caudatus  and 
the  nucleus  lenticularis ,  the  caudate  nucleus,  with  the  growth  of 
the  cerebral  hemisphere,  being  prolonged  backwards  as  a  slender 
process  almost  to  the  tip  of  lateral  horn  of  the  ventricle.  From 
the  dorsal  portions  of  the  dorsal  zones  of  the  telencephalon  the 
remaining  or  mantle  (pallial)  portions  of  the  hemispheres  are 
developed  (Figs.  246,  h  and  238,  F14).  When  first  formed,  the 
hemispheres  are  slight  evaginations  from  the  median  portion  of 
the  telencephalon  the  openings  by  which  their  cavities  communi- 
cate with  the  third  ventricle,  the  interventricular  foramina,  being 
relatively  very  large  (Fig.  246),  but,  in  later  stages  (Fig.  238), 


or  ir 

Fig,  246. — Median  Longitudinal   Section   of  the   Brain   of  an   Embryo  of 

16.3  MM. 

hr.  Anterior  brachium;  eg,  corpus  geniculatum  laterale;  cs,  corpus  striatum;  IC, 
cerebral  hemisphere;  ir,  infundibular  recess;  It,  lamina  terminalis;  or,  optic  recess;  ol, 
thalamus;  p,  pineal  process;  sm,  sulcus  Monroi;  st,  hypothalamic  region;  vi,  velum 
interpositum. — {His.) 

the  hemispheres  increase  more  markedly  and  eventually  surpass 
all  the  other  portions  of  the  brain  in  magnitude,  overlapping 
and  completely  concealing  the  roof  and  sides  of  the  diencephalon 
and  mesencephalon  and  also  the  anterior  surface  of  the  cerebellum. 
In  this  enlargement,  however,  the  interventricular  foramina  share 
only  to  a  slight  extent  and  consequently  become  relatively  smaller 
(Fig.  238),  forming  in  the  adult  merely  slit-like  openings  lying 
between  the  lamina  terminalis  and  the  thalami  and  having  for 
their  roof  the  anterior  portion  of  the  velum  interpositum. 


4o6 


THE    TELENCEPHALON 


The  velum  interpositum — that  is  to  say,  the  roof-plate — 
where  it  forms  the  roof  of  the  interventricular  foramen,  is  pro- 
longed out  upon  the  dorsal  surface  of  each  hemisphere,  and,  be- 
coming invaginated,  forms  upon  it  a  groove.  As  the  hemispheres, 
increasing  in  height,  develop  a  mesial  wall,  the  groove,  which  is 
the  so-called  chorioidal  fissure^  comes  to  lie  along  the  ventral  edge 
of  this  wall,  and  as  the  growth  of  the  hemispheres  continues  it 
becomes  more  and  more  elongated,  being  carried  at  first  backward 
(Fig.  247),  then  ventrally,  and  finally  forward  to  end  at  the  tip 

of  the  temporal  lobe.  After  the  estab- 
lishment of  the  grooves  the  mesenchyme 
in  their  vicinity  dips  into  them,  and, 
developing  blood-vessels,  becomes  the 
chorioid  plexuses  of  the  lateral  ventricles, 
and  at  first  these  plexuses  grow  much 
more  rapidly  than  the  ventricles,  and 
so  fill  them  almost  completely.  Later, 
however,  the  walls  of  the  hemispheres 
gain  the  ascendancy  in  rapidity  of 
growth  and  the  plexuses  become  rela- 
tively much  smaller.  Since  the  por- 
tions of  the  roof -plate  which  form  the 
chorioidal  fissures  are  continuous  with 
the  velum  interpositum  in  the  roofs  of  the  interventricular  foramina, 
the  chorioid  plexuses  of  the  lateral  and  third  ventricles  become 
continuous  also  at  that  point. 

The  mode  of  growth  of  the  chorioid  fissures  seems  to  indicat 
the  mode  of  growth  of  the  hemispheres.     At  first  the  growth 
more  or  less  equal  in  all  directions,  but  later  it  becomes  mori 
extensive  posteriorly,   there  being  more  room  for  expansion  ii 
that  direction,  and  when  further  extension  backward  become 
difficult  the  posterior  extremities  of  the  hemispheres  bend  ventrally 
toward  the  base  of  the  cranium,  and  reaching  this,  turn  for  war 
to  form  the  temporal  lobes.     As  a  result  the  cavities  of  the  hemi- 
spheres, the  lateral  ventricles,  in  addition  to  being  carried  forwar 
to  form  an  anterior  horn,  are  also  carried  backward  and  ventrallj 


Fig.  247. — Median  Longi- 
tudinal Section  of  the  Brain 
OF  AN  Embryo  Calf  of  5  cm. 

cb,  Cerebellum;  cp,  chorioid 
plexus;  cs,  corpus  striatum; /M, 
interventricular  foramen;  in, 
hypophysis;  in,  mid-brain;  oc, 
optic  commissure;  t,  posterior 
part  of  the  diencephalon. — 
(Mihalkovicz.) 


THE  CEREBRAL  CONVOLUTIONS  407 

to  form  the  lateral  or  descending  horn,  and  the  corpus  striatum 
likewise  extends  backward  to  the  tip  of  each  temporal  lobe  as  a 
slender  process  known  as  the  tail  of  the  caudate  nucleus.  In 
addition  to  the  anterior  and  lateral  horns,  the  ventricles  of  the 
human  brain  also  possess  posterior  horns  extending  backward 
into  the  occipital  portions  of  the  hemispheres,  these  portions,  on 
account  of  the  greater  persistence  of  the  mid-brain  flexure  (see 
p.  392),  being  enabled  to  develop  to  a  greater  extent  than  in  the 
lower  mammals. 

The  scheme  of  the  origin  of  parts  in  the  telencephalon  may  be 
stated  as  follows: 

Median  Part  Hemisphere 
P     -    ,                                      ( Anterior  part  of  velum  inter-  f  Floor  of    chorioidal 
I   positum.                                     \  fissure. 


positi 

Dorsal  zones |  Lamina  terminalis. 

^  Optic  evaginations. 
Anterior  part  of  hypothalamic 
Ventral  zones \    region. 

[  Anterior  part  of  tuber  cinereum, 


Pallium. 

Corpus  striatum. 
Olfactory  lobes  (see 
p.  411). 


The  Convolutions  of  the  Hemispheres. — The  growth  of  the 
hemispheres  to  form  the  voluminous  structures  found  in  the  adult 
depends  mainly  upon  an  increase  of  size  of  the  pallium.  The 
corpus  striatum,  although  it  takes  part  in  the  elongation  of  each 
hemisphere,  nevertheless  does  not  increase  in  other  directions  as 
rapidly  and  extensively  as  the  pallium,  and  hence,  even  in  very 
early  stages,  a  depression  appears  upon  the  surface  of  the  hemi- 
sphere where  the  corpus  is  situated  (Fig.  248).  This  depression 
is  the  lateral  cerebral  fossa,  and  for  a  considerable  period  it  is  the 
only  sign  of  inequality  of  growth  on  the  outer  surfaces  of  the 
hemispheres.  Upon  the  mesial  surfaces,  however,  at  about  the 
time  that  the  chorioid  fissure  appears,  another  linear  depression  is 
formed  dorsal  to  the  chorioid,  and  when  fully  formed  extends  from 
in  front  of  the  interventricular  foramen  to  the  tip  of  the  temporal 
lobe  (Fig.  250,  h).  It  affects  the  entire  thickness  of  the  pallial 
wall  and  consequently  produces  an  elevation  upon  the  inner 
surface,  a  projection  into  the  cavity  of  the  ventricle  which  is  known 


4o8 


THE   CEREBRAL   CONVOLUTIONS 


as  the  hippocampus,  whence  the  fissure  may  be  termed  the  hip- 
pocampal  fissure.  The  portion  of  the  pallium  which  intervenes 
between  this  fissure  and  the  chorioidal  forms  what  is  known  as  the 
dentate  gyrus. 

Toward  the  end  of  the  third  or  the  beginning  of  the  fourth 
month  two  prolongations  arise  from  the  fissure  just  where  it 
turns  to  be  continued  into  the  temporal  lobe,  and  these,  extending 
posteriorly,  give  rise  to  the  parieto-occipital  and  calcarine  fissures. 
Like  the  hippocampal,  these  fissures  produce  elevations  upon  the 

inner  surface  of  the  pallium, 
that  formed  by  the  parieto- 
occipital early  disappearing, 
while  that  produced  by  the 
calcarine  persists  to  form  the 
calcar  (hippocampus  minor)  of 
adult  anatomy. 

The  three  fissures  just  de- 
scribed, together  with  the 
chorioidal  and  the  lateral 
cerebral  fossa,  are  all  formed 
by  the  beginning  of  the  fourth 
month  and  all  the  fissures 
affect  the  entire  thickness  of 
the  wall  of  the  hemisphere, 
and  hence  have  been  termed  the  primary  or  total  fissures.  Unti 
the  beginning  of  the  fifth  month  they  are  the  only  fissures  presentj 
but  at  that  time  secondary  fissures,  which,  with  one  exceptionj 
are  merely  furrows  of  the  surface  of  the  pallium,  make  thei 
appearance  and  continue  to  farm  until  birth  and  possibly  later 
Before  considering  these,  however,  certain  changes  which  occur  ii 
the  neighborhood  of  the  lateral  cerebral  fossa  may  be  described. 
The  fossa  is  at  first  a  triangular  depression  situated  above  th( 
temporal  lobe  on  the  surface  of  the  hemisphere.  During  th( 
fourth  month  it  deepens  considerably,  so  that  its  upper  and  lowei 
margins  become  more  pronounced  and  form  projecting  folds,  and 
during  the  fifth  month,  these  two  folds  approach  one  another  anc 


Fig.  248. — Brain  of  an  Embryo  of  the 

Fourth  Month. 

c,  Cerebellum;  p,  pons;  s,  lateral  cerebral 

fossa. 


THE  CEREBRAL  CONVOLUTIONS 


409 


eventually  cover  in  the  floor  of  the  fossa  completely,  the  groove 
which  marks  the  line  of  their  contact  forming  the  lateral  cerebral 
fissure,  while  the  floor  of  the  fossa  becomes  known  as  the  insula. 
The  first  of  the  secondary  fissures  to  appear  is  the  sulcus  cinguli, 
which  is  formed  about  the  middle  of  the  fifth  month  on  the  mesial 
surface  of  the  hemispheres,  lying  parallel  to  the  anterior  portion 
of  the  hippocampal  fissure  and  dividing  the  mesial  surface  into 
the  gyri  marginalis  snidfornicatus.  A  little  later,  at  the  beginning 
of  the  sixth  month,  several  other  fissures  make  their  appearance 


Fig.  249. — Cerebral  Hemisphere  of  an  Embryo  of  about  the  Seventh  Month. 
fs,  Superior  frontal  sulcus;  ip,  interparietal;  IR,  insula;  pci,  inferior  pre-central; 
pes,  superior  pre-central;  ptc,  post-central;  R,  central;  S,  lateral;  t^,  first  temporal. — 
(Cunningham.) 


upon  the  outer  surface  of  the  pallium,  the  chief  of  these  being  the 
central  sulcus,  the  inter- parietal,  the  pre-  and  post-central,  and  the 
temporal  sulci,  the  most  ventral  of  these  last  running  parallel  with 
the  lower  portion  of  the  hippocampal  fissure  and  differing  from  the 
others  in  forming  a  ridge  on  the  wall  of  the  ventricle  termed  the 
collateral  eminence,  whence  the  fissure  is  known  as  the  collateral. 
The  position  of  most  of  these  fissures  may  be  seen  from  Fig.  249, 
and  for  a  more  complete  description  of  them  reference  may  be  had 
to  text-books  of  descriptive  anatomy. 


4IO  THE   CORPUS    CALLOSUM 

In  later  stages  numerous  tertiary  fissures  make  their  appear- 
ance and  mask  more  or  less  extensively  the  secondaries,  than  which 
they  are,  as  a  rule,  much  more  inconstant  in  position  and  shallower. 

The  Corpus  Callosum  and  Fornix. — While  these  fissures  have 
been  forming,  important  structures  have  developed  in  connection 
with  the  lamina  terminalis.  Up  to  about  the  fourth  month  the 
lamina  is  thin  and  of  nearly  uniform  thickness  throughout,  but  at 
this  time  it  begins  to  thicken  near  its  dorsal  edge  and  fibers  appear 
in  the  thickening.  These  fibers  belong  to  three  sets.  In  the 
first  place,  certain  of  them  arise  in  connection  with  the  olfactory 
tracts  (see  p.  412)  and  from  the  region  of  the  hippocampal  gyrus, 
which  is  also  associated  with  the  olfactory  sense,  and,  passing 
through  the  substance  of  the  lamina  terminalis,  they  extend  across 
the  median  line  to  the  corresponding  regions  of  the  opposite 
cerebral  hemisphere.  They  are  therefore  commissural  fibers  and 
form  what  is  termed  the  anterior  commissure  (Figs.  250,  ca  and 
251,  ac).  Secondly,  fibers,  wh^'ch  have  their  origin  from  the  cells 
of  the  hippocampus,  develop  along  the  chorioidal  edge  of  that 
structure,  forming  what  is  termed  the  fimbria.  They  follow  along 
the  edge  of  the  chorioidal  fissure  and,  when  this  reaches  the 
interventricular  foramen,  they  enter  as  the  pillars  of  the  fornix 
(Figs.  250,  cf\  Fig.  256,/)  the  substance  of  the  lamina  terminalis 
and,  passing  ventrally  in  it,  eventually  reach  the  hypothalamic 
region,  where  they  terminate  in  the  corpora  mammillaria. 

Thirdly,  as  the  mantle  develops  fibers  radiate  from  all  parts  of 
it  toward  the  dorsal  portion  of  the  lamina  terminalis  and  traversing 
it  are  distributed  to  the  corresponding  portions  of  the  mantle  of  the 
opposite  side.  These  fibers  are  also  commissural  in  character  and 
form  the  corpus  callosum  (Figs.  250  and  251,  cc).  With  the  de- 
velopment of  these  three  sets  of  fibers  and  especially  those  forming 
the  corpus  callosum,  the  dorsal  portion  of  the  lamina  terminalis 
becomes  enlarged  so  as  to  form  a  triangular  area  extending  between 
the  two  cerebral  hemispheres  (Fig.  251),  the  corpus  callosum  form- 
ing its  dorsal  portion  and  base,  which  is  directed  anteriorly,  the 
pillars  of  the  fornix  its  ventral  portion,  while  the  anterior  com- 
missure occupies  its  ventral  anterior  angle. 


THE    OLFACTORY    LOBES  4II 

The  portion  of  the  triangle  included  between  the  callosum  and 
the  fornix  remains  thin  and  forms  the  septum  pellucidum,  and  a 
split  occurring  in  the  center  of  this  gives  rise  to  the  so-called  fifth' 
ventricle,  which,  from  its  mode  of  formation,  is  a  completely 
closed  cavity  and  is  not  lined  with  ependymal  tissue  of  the  same 
nature  as  that  found  in  the  other  ventricles. 

Owing  to  the  very  consider- 
able size  reached  by  the  tri- 
angular area  whose  history  has 
just  been  described,  important 
changes  are  wrought  in  the 
adjoining  portions  of  the  mesial 
surface  of  the  hemispheres. 
Before  the  development  of  the 
area  the  gyrus  dentatus  and  the 
hippocampus  extend  forward 
into  the  anterior  portion  of  the 
hemispheres  (Fig.  250),  but  on 
account  of  their  position  they 
become  encroached  upon  by  the        „ 

^  •'  Fig.    250. —  Median    Longitudinal 

enlargement  of   the  corpus  cal-    Section  of  the  Brain  of  an  Embryo  of 

losum,   with   the  result  that  the  ^°'',!' ^aklrrne  fissure;   ca,  anterior  com- 

hippocampus      becomes      p  r  a  C-  "fissure;  u,  corpus  callosum;  c/.  chorioidal 

.  fissure;  dg,   dentate  gyrus;  jm,  interven- 

tlCally  obliterated    m    that  por-  tricularforamen;fe,hippocampalfissure;^o, 

tion  of    its  course  which  lies  in  P^^ieto-occipitalfissure.-(M.-;.a/feo..-c2.) 

the  region  occupied  by  the  corpus  callosum,  its  fissure  in  this  region 
becoming  known  as  the  callosal  fissure,  while  the  corresponding 
portions  of  the  dentate  gyrus  become  reduced  to  narrow  and  insig- 
nificant bands  of  nerve  tissue  which  rest  upon  the  upper  surface  of 
the  corpus  callosum  and  are  known  as  the  lateral  longitudinal  stricB. 
The  Olfactory  Lobes. — ^At  the  time  when  the  cerebral  hemi- 
spheres begin  to  enlarge — that  is  to  say,  at  about  the  fourth  week — 
a  slight  furrow,  which  appears  on  the  ventral  surface  of  each  ante- 
riorly, marks  off  an  area  which,  continuing  to  enlarge  with  the 
hemispheres,  gradually  becomes  constricted  off  from  them  to 
form  a  distinct  lobe-like  structure,  the  olfactory  lobe  (Fig.    238, 


412  HISTOGENESIS    OF   THE    CEREBRAL   CORTEX 

VI  3).  In  most  of  the  lower  mammalia  these  lobes  reach  a  very 
considerable  size,  and  consequently  have  been  regarded  as  con- 
stituting an  additional  division  of  the  brain,  known  as  the  rhinen- 
cephalon,  but  in  man  they  remain  smaller,  and  although  they  are 
at  first  hollow,  containing  prolongations  from  the  lateral  ventricles, 
the  cavities  later  on  disappear  and  the  lobes  become  solid.  Each 
lobe  becomes  differentiated  into  two  portions,  its  terminal  portion 
becoming  converted  into  the  club-shaped  structure,  the  olfactory 

bulb  and  stalk,  while  its 
proximal  portion  gives  rise  to 
the  olfactory  tracts,  the 
trigone,  and  the  anterior 
perforated  substance. 

Histogenesis  of  the  Cere- 
bral Cortex. — In  embryos  of 
the  sixth  week  the  walls  of 
the  cerebral  hemispheres 
present  the  fundamental 
structural   plan  already  de- 

FiG.  251.— Median  Longitudinal   Sec-    scribed    (p.    38 1),    possessing 

TION    OF    THE    BRAIN   OF   AN    EmBRYO   OF   THE  ,  .         , 

Fifth  Month.  an    outer  margmal    zone,   a 

ac.  Anterior  commissure;  cc,  corpus  cal-    middle     mantle    ZOnC  and  an 
losum;  rf|:,  dentate  gyrus; /.fornix ;  j,  infundib-     .  j  i  1    - 

uium;  mc,   intermediate  mass;  si,  septum  mner  ependymal  or    matrix 
?,^.w'^,?''^'^f '   "''  "'^^'''^  interpositum.-   ^onc.     Toward    the    close 

{Mthalkovtcz.) 

of  the  second  month  of 
development  there  begins  a  migration  of  neuroblasts  from  the 
mantle  and  matrix  zones  toward  the  surface,  and  these  migrating 
cells  eventually  come  to  rest  in  the  deeper  layers  of  the  marginal 
zone,  forming  there  a  well-defined  cortical  layer.  This  migration 
becomes  very  pronounced  during  the  third  month  of  development 
and  later  diminishes,  although  it  seems  probable  that  it  is  con- 
tinued in  less  degree  even  until  after  birth  (Melius).  The  neuro- 
blasts of  the  cortical  layer  thus  formed  differentiate  into  the 
pyramidal  and  other  cells  of  the  adult  cortex.  The  outer  layers 
of  the  marginal  zone  become  the  molecular  layer,  and  beneath 
the  cortical  layer  the  axis  cylinders  passing  to  and  from  the 


THE    SPINAL   NERVES  413 

cortical  cells  gradually  form  the  white  substance  of  the  pallium. 
The  fibers  of  the  white  substance  do  not  begin  to  acquire  their 
myelin  sheaths  until  toward  the  end  of  the  ninth  month  and  the " 
process  is  not  completed  until  some  time  after  birth  (Flechsig); 
while  the  fibers  in  the  cortex  continue  to  undergo  myelination  until 
comparatively  late  in  life  (Kaes). 

The  Development  of  the  Spinal  Nerves. — It  has  already 
been  seen  that  there  is  a  fundamental  difference  in  the  mode  of 
development  of  the  two  roots  of  which  the  typical  spinal  nerves 
are  composed,  the  ventral  root  being  formed  by  axis-cylinders 
which  arise  from  neuroblasts  situated  within  the  substance  of 
the  spinal  cord,  while  the  dorsal  roots  arise  from  the  cells  of  the 
neural  crests,  their  axis-cylinders  growing  into  the  substance  of  the 
cord  while  their  dendrites  become  prolonged  peripherally  to 
form  the  sensory  fibers  of  the  nerves.  Throughout  the  thoracic 
lumbar  and  sacral  regions  of  the  cord  the  fibers  which  grow  out 
from  the  anterior  horn  cells  converge  to  form  a  single  nerve-root 
in  each  segment,  but  in  the  cervical  region  fibers  which  arise  from 
the  more  laterally  situated  neuroblasts  make  their  exit  from  the 
cord  independently  of  the  more  ventral  neuroblasts  and  form  the 
roots  of  the  spinal  accessory  nerve  (see  p.  421).  In  the  cervical 
region  there  are  accordingly  three  sets  of  nerve-roots,  the  dorsal, 
lateral,  and  ventral  sets. 

In  a  typical  spinal  nerve,  such  as  one  of  the  thoracic  series,  the 
dorsal  roots  as  they  grow  peripherally  pass  ventrally  as  well  as 
outward,  so  that  they  quickly  come  into  contact  with  the  ventral 
roots  with  whose  fibers  they  mingle,  and  the  mixed  nerve  so 
formed  soon  after  divides  into  two  trunks,  a  dorsal  one,  which  is 
distributed  to  the  dorsal  musculature  and  integument,  and  a  larger 
ventral  one.  The  ventral  division  as  it  continues  its  outward 
growth  soon  reaches  the  dorsal  angle  of  the  pleuro-peritoneal 
cavity,  where  it  divides,  one  branch  passing  into  the  tissue  of  the 
body-wall  while  the  other  passes  into  the  splanchnic  mesoderm. 
The  former  branch,  continuing  its  onward  course  in  the  body-wall, 
again  divides,  one  branch  becoming  the  lateral  cutaneous  nerve, 
while  the  other  continues  inward  to  terminate  in  the  median 


414  THE    CRANIAL   KERVES 

ventral  portion  of  the  body  as  the  anterior  cutaneous  nerve.  The 
splanchnic  branch  forms  a  ramus  communicans  to  the  sympathetic 
system  and  will  be  considered  more  fully  later  on. 
^  The  conditions  just  described  are  those  which  obtain  through- 
out the  greater  part  of  the  thoracic  region.  Elsewhere  the  fibers 
of  the  ventral  divisions  of  the  nerves  as  they  grow  outward  tend 
to  separate  from  one  another  and  to  become  associated  with  the 
fibers  of  adjacent  nerves,  giving  rise  to  plexuses.  In  the  regions 
where  the  limbs  occur  the  formation  of  the  plexuses  is  also  asso- 
ciated with  a  shifting  of  the  parts  to  which  the  nerves  are  supplied, 
a  factor  in  plexus  formation  which  is,  however,  much  more  evident 
from  comparative  anatomical  than  from  embryological  studies. 

The  Development  of  the  Cranial  Nerves. — During  the  last 
thirty  years  the  cranial  nerves  have  received  a  great  deal  of  atten- 
tion in  connection  with  the  idea  that  an  accurate  knowledge 
of  their  development  would  afford  a  clue  to  a  most  vexed  problem 
of  vertebrate  morphology,  the  metamerism  of  the  head.  That  the 
metamerism  which  was  so  pronounced  in  the  trunk  should  extend 
into  the  head  was  a  natural  supposition,  strengthened  by  the  dis- 
covery of  head-cavities  in  the  lower  vertebrates  and  by  the  in- 
dications of  metamerism  seen  in  the  branchial  arches,  and  the 
problem  which  presented  itself  was  the  correlation  of  the  various 
structures  belonging  to  each  metamere  and  the  determination  of 
the  modifications  which  they  had  undergone  during  the  evolution 
of  the  head. 

In  the  trunk  region  a  nerve  forms  a  conspicuous  element  of 
each  metamere  and  is  composed,  according  to  what  is  known  as 
Bell's  law,  of  a  ventral  or  efferent  and  a  dorsal  or  afferent  root. 
Until  comparatively  recently  the  study  of  the  cranial  nerves  has 
been  dominated  by  the  idea  that  it  was  possible  to  extend  the 
application  of  Bell's  law  to  them  and  to  recognize  in  the  cranial 
region  a  number  of  nerve  pairs  serially  homologous  with  the  spinal 
nerves,  some  of  them,  however,  having  lost  their  afferent  roots, 
while  in  others  a  dislocation,  as  it  were,  of  the  two  roots  had 
occurred. 

The  results  obtained  from  investigation  along  this  line  have 


THE   CRANIAL   NERVES  415 

not,  however,  proved  entirely  satisfactory,  and  facts  have  been 
elucidated  which  seem  to  show  that  it  is  not  possible  to  extend 
Bell's  law,  in  its  usual  form  at  least,  to  the  cranial  nerves.  It  has 
been  found  that  it  is  not  sufficient  to  recognize  simply  afferent 
and  efferent  roots,  but  these  must  be  analyzed  into  further  com- 
ponents, and  when  this  is  done  it  is  found  that  in  the  series  of 
cranial  nerves  certain  components  occur  which  are  not  represented 
in  the  nerves  of  the  spinal  series. 

Before  proceeding  to  a  description  of  these  components  it  will 
be  well  to  call  attention  to  a  matter  already  alluded  to  in  a  pre- : 
vious  chapter  (p.  87)  in  connection  with  the  segmentation  of  the 
mesoderm  of  the  head.  It  has  been  pointed  out  that  while  there 
exist  ''head-cavities"  which  are  serially  homologous  with  the 
mesodermal  somites  of  the  trunk,  there  has  been  imposed  upon 
this  primary  cranial  metamerism  a  secondary  metamerism  repre- 
sented by  the  branchiomeres  associated  with  the  branchial  arches, 
and,  it  may  be  added,  this  secondary  metamerism  has  become  the 
more  prominent  of  the  two,  the  primary  one,  as  it  developed, 
gradually  slipping  into  the  background  until,  in  the  higher  verte- 
brates, it  has  become  to  a  very  considerable  extent  rudimentary. 
In  accordance  with  this  double  metamerism  it  is  necessary  to 
recognize  two  sets  of  cranial  muscles,  one  derived  from  the  cranial 
myotomes  and  represented  by  the  muscles  of  the  eyeball,  and  one 
derived  from  the  branchiomeric  mesoderm,  and  it  is  necessary  also 
to  recognize  for  these  two  sets  of  muscles  two  sets  of  motor  nerves, 
so  that,  with  the  dorsal  or  sensory  nerve-roots,  there  are  alto- 
gether three  sets  of  nerve-roots  in  the  cranial  region  instead  of 
only  two,  as  in  the  spinal  region. 

These  three  sets  of  roots  are  readily  recognizable  both  in  the 
embryonic  and  in  the  adult  brain,  especially  if  attention  be  directed 
to  the  cell  groups  or  nuclei  with  which  they  are  associated  (Fig. 
252).  Thus  there  can  be  recognized:  (i)  a  series  of  nuclei  from 
which  nerve-fibers  arise,  situated  in  the  floor  of  the  fourth  ventricle 
and  aquaeduct  close  to  the  median  line  and  termed  the  ventral 
motor  nuclei;  (2)  a  second  series  of  nuclei  of  origin,  situated  more 
laterally  and  in  the  substance  of  the  formatio  reticularis,  and 


4l6  THE    CRANIAL   NERVES 

known  as  the  lateral  motor  nuclei;  and  (3)  a  series  of  nuclei  in 
which  afferent  nerve-fibers  terminate,  situated  still  more  laterally 
in  the  floor  of  the  ventricle  and  forming  the  dorsal  or  sensory 
nuclei.  None  of  the  twelve  cranial  nerves  usually  recognized  in 
the  text-books  contains  fibers  associated  with  all  three  of  these 
nuclei;  the  fibers  from  the  lateral  motor  nuclei  almost  invariably 
unite  with  sensory  fibers  to  form  a  mixed  nerve,  but  those  from  all 


Pig.  252. — Transverse  Section  through  the  Medulla  Oblongata  of  an 
Embryo  of  io  mm.,  showing  the  Nuclei  of  Origin  of  the  Vagus  (X)  and  Hypo- 
glossal  (XII)  NERVES. — (His.) 

the  ventral  motor  nuclei  form  independent  roots,  while  the  ol- 
factory and  auditory  nerves  alone,  of  all  the  sensory  roots  (omit- 
ting for  the  present  the  optic  nerve),  do  not  contain  fibers  from 
either  of  the  series  of  motor  nuclei.  The  relations  of  the  various 
cranial  nerves  to  the  nuclei  may  be  seen  from  the  following  table, 
in  which  the  +  sign  indicates  the  presence  and  the  —  sign  the 
absence  of  fibers  from  the  nuclear  series  under  which  it  stands. 

Two   nerves — namely,    the  second   and   twelfth — have  been 
omitted  from  the  following  table.     Of  these,  the  second  or  optic 


THE    CRANIAL    NERVES 


417 


Number 

Name 

Ventral  Motor 

Lateral  Motor 

Sensory 

I. 

Olfactory. 



+^    ~~^ 

III. 

Oculomotor. 

+ 

— 

— 

IV. 

Trochlear. 

+ 

— 

— 

V. 

Trigeminus. 

~" 

+ 

+ 

VI. 

Abducens. 

+ 

— 

— 

VII. 

Facial. 

— 

+ 

+ 

VIII. 

Auditory. 

- 

- 

+ 

IX. 

Glossopharyngeal. 

— 

'+ 

+ 

X. 

XL 

Vagus                        1 
Spinal  Accessory,     j 

- 

+ 

+ 

L 


nerve  undoubtedly  belongs  to  an  entirely  different  category  from 
the  other  peripheral  nerves,  and  will  be  considered  in  the  following 
chapter  in  connection  with  the  sense-organ  with  which  it  is 
associated  (see  especially  p.  465).  The  twelfth  or  hypoglossal 
nerve,  on  the  other  hand,  really  belongs  to  the  spinal  series  and 
has  only  secondarily  been  taken  up  into  the  cranial  region  in  the 
higher  vertebrates.  It  has  already  been  seen  (p.  172)  that  the 
bodies  of  four  vertebrae  are  included  in  the  basioccipital  bone,  and 
that  three  of  the  nerves  corresponding  to  these  vertebrae  are  rep- 
resented in  the  adult  by  the  hypoglossal  and  the  fourth  by  the 
first  cervical  or  suboccipital  nerve.  The  dorsal  roots  of  the  hypo- 
glossal nerves  seem  to  have  almost  disappeared,  although  a 
ganglion  has  been  observed  in  embryos  of  7  and  10  mm.  in  the 
posterior  part  of  the  hypoglossal  region  (His),  and  probably  repre- 
sents the  dorsal  root  of  the  most  posterior  portion  of  the  hypo- 
glossal nerve.  This  ganglion  disappears,  as  a  rule,  in  later  stages, 
and  it  is  interesting  to  note  that  the  ganglion  of  the  suboccipital 
nerve  is  also  occasionally  wanting  in  the  adult  condition. 

An  additional  nerve,  known  as  the  n.  terminalis,  has  been  observed 
both  in  fetal  and  adult  brains.  It  is  quite  small  and  takes  its  origin 
from  the  region  of  the  olfactory  trigone,  whence  it  extends  forward, 
lying  medially  to  the  olfactory  stalk.  It  passes  through  the  cribriform 
plate  of  the  ethmoid  and  is  distributed  in  the  mucous  membrane  of  the 
nasal  septum,  ganglion  cells  occurring  along  its  course.  It  seems  to  be 
quite  distinct  from  the  olfactory  nerve,  but  its  exact  significance  is  as 
yet  somewhat  obscure. 

27 


41 8  THE   CRANIAL   NERVES 

From  what  has  been  said  it  is  evident  that  the  hypoglossal 
roots  are  to  be  regarded  as  equivalent  to  the  ventral  roots  of 
the  cervical  spinal  nerves,  and  the  nuclei  from  which  they  arise 
lie  in  series  with  the  cranial  ventral  motor  roots,  a  fact  which 
indicates  the  equivalency  of  these  latter  with  the  fibers  which 
arise  from  the  neuroblasts  of  the  anterior  horns  of  the  spinal 
cord.  The  equivalents  of  the  lateral  motor  roots  may  more 
conveniently  be  considered  later  on,  but  it  may  be  pointed  out 
here  that  these  are  the  fibers  which  are  distributed  to  the  muscles 
of  the  branchiomeres.  In  the  case  of  the  sensory  nerves  a  further 
analysis  is  necessary  before  their  equivalents  in  the  spinal  series 
can  be  determined.  For  this  the  studies  which  have  been  made 
in  recent  years  of  the  components  entering  into  the  cranial  nerves 
of  the  amphibia  (Strong)  and  fishes  (Herrick)  must  supply  a 
basis,  since  as  yet  a  direct  analysis  of  the  mammalian  nerves  has 
not  been  made.  In  the  forms  named  it  has  been  found  that  three 
different  components  enter  into  the  formation  of  the  dorsal  roots 
of  the  cranial  nerves:  (i)  fibers  belonging  to  a  general  cutaneous 
or  somatic  sensory  system,  distributed  to  the  skin  without  being 
connected  with  any  special  sense-organs;  (2)  fibers  belonging  to 
what  is  termed  the  communis  or  viscerosensory  system,  dis- 
tributed to  the  walls  of  the  mouth  and  pharyngeal  region  and  to 
special  organs  found  in  the  skin  of  the  same  character  as  those 
occurring  in  the  mouth;  and  (3)  fibers  belonging  to  a  special  set 
of  cutaneous  sense-organs  largely  developed  in  the  fishes  and 
known  as  the  organs  of  the  lateral  line. 

The  fibers  of  the  somatic  sensory  system  converge  to  a  group 
of  cells,  situated  in  the  lateral  part  of  the  floor  of  the  fourth 
ventricle  and  forming  what  is  termed  the  trigeminal  lobe,  and  also 
extend  posteriorly  in  the  substance  of  the  medulla  (Fig.  253), 
forming  what  has  been  termed  the  spinal  root  of  the  trigeminus 
and  terminating  in  a  column  of  cells  which  represents  the  forward 
continuation  of  the  posterior  horn  of  the  cord.  In  the  fishes  and 
amphibia  fibers  belonging  to  this  system  are  to  be  found  in  the 
fifth,  seventh,  and  tenth  nerves,  but  in  the  mammalia  their  dis- 
tribution has  apparently  become  more  limited,  being   confined 


THE    CRANIAL   NERVES 


419 


almost  exclusively  to  the  trigeminus,  of  whose  sensory  divisions 
they  form  a  very  considerable  part.  Since  the  cells  around  which 
the  fibers  of  the  spinal  root  of  the  trigeminus  terminate  are  the 
forward  continuations  of  the  posterior  horn  of  the  cord,  it  seems 
probable  that  the  fibers  of  this  system  are  the  cranial  representa- 
tives of  the  posterior  roots  of  the  spinal  nerves,  which,  it  may  be 
noted,  are  also  somatic  in  their  distribution. 

The  fibers  of  the  viscero-sensory  system  are  found  in  the  lower 
forms  principally  in  the  ninth  and  tenth  nerves  (see  Fig.  250), 


nx 


Pig.  253. — Diagrams  showing  the  Sensory  Components  of  the  Cranial  Nerves 

OF  A  Fish  (Menidia). 

The  somatic  sensory  system  is  unshaded,  the  viscero-sensory  is  cross-hatched, 
and  the  lateral  line  system  is  black,  asc.v.  Spinal  root  of  trigeminus;  brx,  branchial 
branches  of  vagus;  Ix,  lobus  vagi;  ol,  olfactory  bulb;  op,  optic  nerve;  rc.x,  cutaneous 
branch  of  the  vagus;  rix,  intestinal  branch  of  vagus;  rl,  lateral  line  nerve;  rl.acc, 
accessory  lateral  line  nerve;  ros,  superficial  ophthalmic;  rp,  ramus  palatinus  of  the 
facial;  thy,  hyomandibular  branch  of  the  facial;  t.inf,  infraorbital  nerve. — {Herrick.) 


although  groups  of  them  are  also  incorporated  in  the  seventh  and 
fifth.  They  converge  to  a  mass  of  cells,  known  as  the  lobus  vagi, 
and  like  the  first  set  are  also  continued  down  the  medulla  to  form 
a  tract  known  as  the  fasciculus  solitarius  or  fasciculus  communis. 
In  the  mammalia  the  system  is  represented  by  the  sensory  fibers 


420  THE    CRANIAL   NERVES 

of  the  glosso-pharyngeo-vagus  set  of  nerves,  of  which  it  repre- 
sents practically  the  entire  mass;  by  the  sensory  fibers  of  the  facial 
arising  from  the  geniculate  ganglion  and  included  in  the  chorda 
tympani  and  probably  also  the  great  superficial  petrosal;  and  also, 
probably,  by  the  lingual  branch  of  the  trigeminus.  Further- 
more, since  the  mucous  membrane  of  the  palate  is  supplied  by 
branches  from  the  trigeminus  which  pass  by  way  of  the  spheno- 
palatine (Meckel's)  ganglion,  and  the  same  region  is  supplied 
in  lower  forms  by  a  palatine  branch  from  the  facial,  it  seems  prob- 
able that  the  palatine  nerves  of  the  mammalia  are  also  to  be  as- 
signed to  this  system.*  If  this  be  the  case,  a  very  evident  clue  is 
afforded  to  the  homologies  of  the  system  in  the  spinal  nerves,  for 
since  the  spheno-palatine  ganglion  is  to  be  regarded  as  part  of 
the  sympathetic  system,  the  sensory  fibers  which  pass  from  the 
viscera  to  the  spinal  cord  by  way  of  the  sympathetic  system  (p. 
425)  present  relations  practically  identical  with  those  of  the 
palatine  nerves. 

Finally,  with  regard  to  the  system  of  the  lateral  line,  there 
seems  but  little  doubt  that  it  has  no  representation  whatsoever  in 
the  spinal  nerves.  It  is  associated  with  a  peculiar  system  of 
cutaneous  sense-organs  found  only  in  aquatic  or  marine  animals, 
and  also  with  the  auditory  and  possibly  the  olfactory  organs,  the 
former  of  which  are  certainly  and  the  latter  possibly  primarily 
parts  of  the  lateral  line  system  of  organs.  The  organs  are  prin- 
cipally confined  to  the  head,  although  they  also  extend  upon  the 
trunk,  where  they  are  followed  by  a  branch  from  the  vagus  nerve, 
the  entire  system  being  accordingly  supplied  by  cranial  nerves. 
In  the  fishes,  in  which  the  development  of  the  organs  is  at  a 
maximum,  fibers  belonging  to  the  system  are  found  in  all  the 
branchiomeric  nerves  and  all  converge  to  a  portion  of  the  medulla 
known  as  the  tuberculum  acusticum.     In  the  Mammalia,  with  the 

*  The  fact  that  the  palatine  branches  are  associated  with  the  trigeminus  in  the 
Mammalia  and  with  the  facial  in  the  Amphibia  is  readily  explained  by  the  fact  that 
in  the  latter  the  Gasserian  and  geniculate  ganglia  are  not  always  separated,  so  that 
it  is  possible  for  fibers  originating  from  the  compound  ganglion  to  pass  into  either 


THE    CRANIAL    NERVES 


421 


disappearance  of  the  lateral  line  organs  there  has  been  a  dis- 
appearance of  the  associated  nerves,  and  the  only  certain  repre- 
sentative of  the  system  which  persists  is  the  auditory  nerve. 

The  table  given  on  p.  417  may  now  be  expanded  as  follows, 
though  it  must  be  recognized  that  such  an  analysis  of  the  mam- 
malian nerves  is  merely  a  deduction  from  what  has  been  observed 
in  lower  forms,  and  may  require  some  modifications  when  the 
components  have  been  subjected  to  actual  observations: 


Nerve 

Ventral 
Motor 

Lateral 
Motor 

Somatic 
Sensory 

Visceral 
Sensory 

Lateral 
Line 

I. 

__ 







+ 

III. 

4- 

— 

- 

- 

— 

IV. 

+ 

— 

— 

— 

— 

V. 

— 

+ 

+ 

+ 

— 

VI 

+ 

— 

— 

— 

— 

VII. 

— 

+ 

— 

+ 

— 

VIII. 

— 

— 

— 

— 

+ 

IX.  ^ 

X. 

- 

+ 

+ 

+ 

- 

XI.  ] 

XII. 

+ 

— 

— 

— 

— 

Spinal. 

+ 

(?) 

+ 

+ 

— 

An  additional  word  is  necessary  concerning  the  spinal  accessory 
nerve,  for  it  presents  certain  interesting  relations  which  possibly 
furnish  a  clue  to  the  spinal  equivalents  of  the  lateral  motor  roots. 
In  the  first  place,  neuroblasts  which  give  rise  to  those  fibers  of  the 
nerve  which  come  from  the  spinal  cord  are  situated  in  the  dorsal 
part  of  the  ventral  zones.  As  the  nuclei  of  origin  are  traced 
anteriorly  they  will  be  found  to  change  their  position  somewhat 
as  the  medulla  is  reached  and  eventually  come  to  lie  in  the  reticular 
formation,  the  most  anterior  of  them  being  practically  continuous 
with  the  motor  nucleus  of  the  vagus.  Indeed,  it  seems  that  the 
spinal  accessory  nerve  is  properly  to  be  regarded  as  an  extension 
of  the  vagus  downward  into  the  cervical  region  (Fiirbringer, 
Streeter),  a  process  which  reaches  its  greatest  development  in 


422  THE   CRANIAL   NERVES 

the  mammalia  and  seems  to  stand  in  relation  to  the  development 
of  those  portions  of  the  trapezius  and  sterno-mastoid  muscles 
which  are  supplied  by  the  spinal  accessory  nerve. 

It  is  believed  that  the  white  rami  communicantes  which  pass 
from  the  spinal  cord  to  the  thoracic  and  upper  lumbar  sym- 
pathetic ganglia  arise  from  cells  situated  in  the  dorso-lateral  por- 
tions of  the  ventral  horns,  and  it  is  noteworthy  that  white  rami 
are  wanting  in  the  region  in  which  the  spinal  accessory  nerve 
occurs.  Since  this  nerve  represents  a  cranial  lateral  motor  root 
the  temptation  is  great  to  regard  the  cranial  lateral  motor  roots 
as  equivalent  to  the  white  rami  of  the  cord,  and  this  temptation 
is  intensified  when  it  is  recalled  that  there  are  both  embryological 
and  topographical  reasons  for  regarding  the  branchiomeric 
muscles,  to  which  the  cranial  lateral  motor  nerves  are  supplied,  as 
equivalent  to  the  visceral  muscles  of  the  trunk.  But  in  view  of  the 
fact  that  a  sympathetic  neurone  is  always  interposed  between  a 
white  ramus  fiber  and  the  visceral  musculature  (see  Fig.  255), 
while  the  lateral  motor  fibers  connect  directly  with  the  branchio- 
meric musculature,  it  seems  advisable  to  await  further  studies 
before  yielding  to  the  temptation. 

As  regards  the  actual  development  of  the  cranial  nerves,  they 
follow  the  general  law  which  obtains  for  the  spinal  nerves,  the 
motor  fibers  being  outgrowths  from  neuroblasts  situated  in  the 
walls  of  the  neural  tube,  while  the  sensory  nerves  are  outgrowths 
from  the  cells  of  ganglia  situated  without  the  tube.  In  the  lower 
vertebrates  a  series  of  thickenings,  known  as  the  supr abranchial 
placodes,  are  developed  from  the  ectoderm  along  a  line  correspond- 
ing with  the  level  of  the  auditory  invagination,  while  on  a  line 
corresponding  with  the  upper  extremities  of  the  branchial  clefts 
another  series  occurs  which  has  been  termed  that  of  the  epi- 
branchial  placodes,  and  with  both  of  these  sets  of  placodes  the 
cranial  nerves  are  in  connection.  In  the  human  embryo  epi- 
branchial  placodes  have  been  found  in  connection  with  the  fifth, 
seventh,  ninth  and  tenth  nerves,  to  whose  ganglia  they  contribute 
cells.  The  suprabranchial  placodes,  which  in  the  lower  verte- 
brates are  associated  with  the  lateral  line  nerves,  are  unrepresented 


THE    SYMPATHETIC    SYSTEM  423 

in  man,  unless,  as  has  been  maintained,  the  sense-organs  of  the 
internal  ear  are  their  representatives. 

From  what  has  been  said  above  it  is  clear  that  the  usual  arrangement 
of  the  cranial  nerves  in  twelve  pairs  does  not  represent  their  true  relation- 
ships with  one  another.  The  various  pairs  are  serially  homologous 
neither  with  one  another  nor  with  the  typical  spinal  nerves,  nor  can  they 
be  regarded  as  representing  twelve  cranial  segments.  Indeed,  it  would 
seem  that  comparatively  little  information  with  regard  to  the  number  of 
myotomic  segments  which  have  fused  together  to  form  the  head  is  to  be 
derived  from  the  cranial  nerves,  for  while  there  are  only  four  of  these 
nerves  which  are  associated  with  structures  equivalent  to  the  meso- 
dermic  somites  of  the  trunk,  a  much  greater  number  of  head  cavities  or 
mesodermic  somites  has  been  observed  in  the  cranial  region  of  the 
embryos  of  the  lower  vertebrates,  Dohrn,  for  instance,  having  found  nine- 
teen and  Killian  eighteen  in  the  cranial  region  of  Torpedo.  Further- 
more, it  is  not  possible  to  say  at  present  whether  the  branchiomeres  and 
their  associated  nerves  correspond  with  one  or  several  of  the  cranial 
mesodermic  somites,  or  whether,  indeed,  any  correspondence  whatever 
exists. 

In  early  stages  of  development  a  series  of  constrictions  have  been 
observed  in  the  cranial  portion  of  the  neural  tube  and  have  been  re- 
garded as  indicating  a  primitive  segmentation  of  that  structure.  The 
neuromeres,  as  the  intervals  between  successive  constrictions  have  been 
termed,  seem  to  correspond  with  the  cranial  nerves  as  usually  recog- 
nized and  hence  cannot  be  regarded  as  primitive  segmental  structures. 
They  are  more  probably  secondary  and  due  to  the  arrangement  of  the 
neuroblasts  corresponding  to  the  various  nerves. 

The  Development  of  the  Sympathetic  Nervous  System.— 

From  the  embryological  standpoint  the  distinction  which  has  been 
generally  recognized  between  the  sympathetic  and  central  nervous 
systems  does  not  exist,  the  former  having  been  found  to  be  an 
outgrowth  from  the  latter.  This  mode  of  origin  has  been  observed 
with  especial  clearness  in  the  embryos  of  some  of  the  lower  verte- 
brates, in  which  masses  of  cells  have  been  seen  to  separate  from  the 
posterior  root  ganglia  to  form  the  ganglia  of  the  ganglionated  cord 
(Fig.  254).  In  the  mammalia,  including  man,  the  relations  cf  the 
two  sets  of  ganglia  to  one  another  is  by  no  means  so  apparent,  since 
the  sympathetic  cells,  instead  of  being  separated  from  the  posterior 
root  ganglion  en  masse,  migrate  from  it  singly  or  in  groups,  and  are 
therefore  less  readily  distinguishable  from  the  surrounding  meso- 
dermal tissues. 


4^4 


THE   SYMPATHETIC   SYSTEM 


Flc  254. — ^TlLAJBmESK  Smliiuw  through  an  Embkto  Shark:  (Scynimm)  of  1$  mm. 

imumtmM  tbb  Okksdt  or  a  Syxpathstk  Ganglion. 
Ck,  IfotodMvd;  £,  ectoderm;  G,  portenor  root  gang1if>n;  G5.  sympathetic  gangHon; 
Jf .  spmal  cord. — (CHwrfi.) 


THE   SYMPATHETIC  STSTEM 


425 


To  nndrrtfand  the  devdfapmtat  ai  liie  sywpathrtic  sytUat,  it 
must  be  remembered  that  it  conssts  typkaDj  ai  tkree  sets  ol 
glia.  Oneof  thcseiscoastitatcdbytheguigjlEioftkej 
cord  (Fig.  255,  GC),  the  secood  is  icpnscBted  by  die  gweH»  ^  tibe 
pr2ef\-eTtebrai  plezoses  (FVG),  sodi  as  Ae  caardiac^  adku,  l^fpogas- 
trie,  and  pehic,  wliile  die  tldni  or  pcr^iicral  set  (jPG)  is  formed  bj 
the  cells  wiiidi  occur  thiooj^iOBt  the  tiwHry  of  piobafalsr  most  of 
the  >isccral  organs^  cfther  m  small  gnmps  or  scattered  thro«g|b 
plexuses  such  as  the  Auerbach  and  MeBsacr  plenwes  of  tibe 
intestine.    Each  cell  in  these  Tarioas  gmefia  stands  in  direct 


I 


contact  with  the  axis-cyiinder  of  a 

nervous  system,  probably  in  the  lateral  ham  of  die 

the  corrc^pooding  ic^omoi  the  brai%  so  that  each 

terminal  link  of  a  chain  whose  hist  fink  is  a  m 

the  central  system  (Huber).     Thuifhiml  die 

lumbar  regions  of  the  body  the  central 

tinct  cords  known  as  the  vkUe  wmad 

ni^j .  which  pass  from  die  spinal  nerrcs  to  dK 

the  gangtinnatcd  oord^  some  of  them  terminating  aionnd  Ae 

of  these  ganelia,  others  passing  oa  to  the  cds  of  the  praevertchial 


426 


THE    SYMPATHETIC    SYSTEM 


ganglia,  and  others  to  those  of  the  peripheral  plexuses.  In  the 
cervical,  lower  lumbar  and  sacral  regions  white  rami  are  wanting, 
the  central  neurones  in  the  first-named  region  probably  making 
their  way  to  the  s>Tnpathetic  cells  by  way  of  the  upper  thoracic 
nerves,  while  in  the  lower  regions  they  may  pass  down  the  gan- 
glionated  cord  from  higher  regions  or  may  join  the  praevertebral 


Pig.  256. — Transverse  Sectiok  through  the  Spinal  Cord  of  an  Embryo  of 

7  MM. 

c,  Notochord;  g,  posterior  root  ganglion;  m,  spinal  cord;  s,  sympathetic  cell  migrating 
from  the  posterior  root  ganglion;  wr,  white  ramus. — {His.) 

and  peripheral  ganglia  directly  without  passing  through  the  proxi- 
mal ganglia.  In  addition  to  these  white  rami,  what  are  known  as 
gray  rami  also  extend  between  the  proximal  ganglia  and  the  spinal 
nerves;  these  are  composed  of  fibers,  arising  from  sympathetic  cells 
which  join  the  spinal  nerves  in  order  to  pass  with  them  to  their 
ultimate  distribution. 

The  brief  description  here  given  applies  especially  to  the  sym- 
pathetic system  of  the  neck  and  trunk.     Representatives  of  the 


THE    SYMPATHETIC    SYSTEM  427 

system  are  also  found  in  the  head,  in  the  form  of  a  series  of 
ganglia  connected  with  the  trigeminal  and  facial  nerves  and  known 
as  the  ciliary,  spheno-palatine,  otic,  and  submaxillary  ganglia  f 
and,  as  will  be  seen  later,  there  are  probably  some  sympathetic 
cells  which  owe  their  origin  to  the  root  ganglia  of  the  vagus  and 
glossopharyngeal  nerves.  There  is  nothing,  however,  in  the 
head  region  corresponding  to  the  longitudinal  bundles  of  fibers 
which  unite  the  various  proximal  ganglia  of  the  trunk  to  form 
the  ganglionated  cord. 

The  first  distinct  indications  of  the  sympathetic  system  are 
to  be  seen  in  a  human  embryo  of  about  7  mm.  As  the  spinal 
nerves  reach  the  level  of  the  dorsal  edge  of  the  body-cavity,  they 
branch,  one  of  the  branches  continuing  ventrally  in  the  body- wall 
while  the  other  (Fig.  256,  ur)  passes  mesially  toward  the  aorta,  some 
of  its  fibers  reaching  that  structure,  while  others  bend  so  as  to 
assume  a  longitudinal  direction.  These  mesial  branches  repre- 
sent the  white  rami  communicantes,  but  as  yet  no  ganglion  cells 
can  be  seen  in  their  course.  The  cells  of  the  posterior  root  ganglia 
have  already,  for  the  most  part,  assumed  their  bipolar  form,  but 
among  them  there  may  still  be  found  a  number  of  cells  in  the 
neuroblast  condition,  and  these  (Fig.  256,  5),  wandering  out  from 
the  ganglia,  give  rise  to  a  column  of  cells  standing  in  relation  to 
the  white  rami.  At  first  there  is  no  indication  of  a  segmental 
arrangement  of  the  cells  of  the  column  (Fig.  257),  but  at  about 
the  seventh  week  such  an  arrangement  makes  its  appearance  in  the 
cerv'ical  region,  and  later,  extends  posteriorly,  until  the  column 
assumes  the  form  of  the  ganglionated  cord. 

This  origin  of  the  ganglionated  cord  from  cells  migrating  out 
from  the  posterior  root  ganglia  has  been  described  by  various 
authors,  but  recently  the  origin  of  the  cells  has  been  carried  a 
step  further  back,  to  the  mantle  layer  of  the  central  nervous 
system  (Kuntz).  Indifferent  cells  and  neuroblasts  are  said  to 
wander  out  from  the  walls  of  the  medullary  canal  by  way  of  both 
the  posterior  and  anterior  roots  and  it  is  claimed  that  these 
are  the  cells  that  give  rise  to  the  ganglionated  cord  in  the  manner 
just  described. 


428 


THE    SYMPATHETIC    SYSTEM 


Before,  however,  the  segmentation  of  the  ganglionated  cord 
becomes  marked,  thickenings  appear  at  certain  regions  of  the  cell 
column,  and  from  these,  bundles  of  fibers  may  be  seen  extending 


Pig.  257. — Reconstruction   of    the   Sympathetic   System   of   an   Embryo   of 

10.2  MM. 
am,  Vitdline  artery;  ao,  aorta;  au,  umbilical  artery;  bg,  ganglionic  mass  repre- 
senting the  i>dvic  plexus;  d.  intestine;  o€,  oesophagus;  pc.  ganglia  of  the  coeliac  plexus; 
ph,  pharynx;  rv,  right  vagus  nerve;  sp,  splanchnic  nerves;  sy,  ganglionated  cord;  t, 
trachea;*  peripheral  sympathetic  ganglia  in  the  walls  of  the  stomach. — {His,  Jr.) 

ventrally  toward  the  viscera.  The  thickenings  represent  certain 
of  the  praevertebral  ganglia,  and  later  cells  wander  out  from  them 
and  take  position  in  front  of  the  aorta.     In  an  embryo  of  10.2 


THE    SYMPATHETIC    SYSTEM  429 

mm.  two  ganglionic  masses -(Fig.  257,  pc)  occur  in  the  vicinity  of 
the  origin  of  the  vitelline  artery  (am),  one  lying  above  and  the 
other  below  that  vessel;  these  masses  represent  the  ganglia  oT 
the  coeliac  plexus  and  have  separated  somewhat  from  the  gang- 
lionated  cord,  the  fiber  bundles  which  unite  the  upper  mass  with 
the  cord  representing  the  greater  and  lesser  splanchnic  nerves 
(sp),  while  that  connected  with  the  lower  mass  represents  the 
connection  of  the  cord  with  the  superior  mesenteric  ganglion. 
Lower  down,  in  the  neighborhood  of  the  umbilical  arteries,  is 
another  enlargement  of  the  cord  (bg),  which  probably  represents 
the  inferior  mesenteric  and  hypogastric  ganglia  which  have  not 
yet  separated  from  the  cell  column. 

With  the  peripheral  ganglia  the  conditions  are  slightly  dif- 
ferent, in  that  they  are  formed  ver>'  largely,  if  not  exclusively, 
from  cells  that  migrate  from  the  walls  of  the  hind-brain  by  way 
of  the  vagus  nerves  (Fig.  257).  In  this  way  the  ganglia  of  the 
myenteric,  pulmonary  and  cardiac  plexuses  are  formed,  though 
in  the  case  of  the  last  named  it  is  probable  that  contributions  are 
also  received  from  the  ganglionated  cord. 

The  elongated  courses  of  the  cardiac  sympathetic  and  splanchnic 
nerves  in  the  adult  receive  an  explanation  from  the  recession  of  the  heart 
and  diaphragm  (see  pp.  240  and  325),  the  latter  process  forcing  down- 
ward the  coeliac  plexus,  which  originally  occupied  a  position  opposite  the 
region  of  the  ganglionated  cord  from  which  the  splanchnic  nerves  arise. 

As  regards  the  cephalic  s}'mpathetic  ganglia,  the  observations 
of  Remak  on  the  chick  and  Kolliker  on  the  rabbit  show  that  the 
ciliar>',  sphenopalatine,  and  otic  ganglia  arise  by  the  separation 
of  cells  from  the  semilunar  (Gasserian)  ganglion,  and  from  their 
adult  relations  it  may  be  supposed  that  the  cells  of  the  submaxillary 
and  sublingual  ganglia  have  similarly  arisen  from  the  geniculate 
ganglion  of  the  facial  nerve.  Evidence  has  also  been  obtained 
from  human  embr}'os  that  s>inpathetic  cells  are  derived  from  the 
ganglia  of  the  vagus  and  glossophar>'ngeal  ner\'es,  but,  instead 
of  forming  distinct  ganglia  in  the  adult,  these,  in  all  probability, 
associate  themselves  with  the  first  cervical  ganglia  of  the  gang- 
lionated cord. 


43  O        ^^^^  LITERATURE 


LITERATURE 


P.  Bailey:  "Morphology  of  the  roof-plate  of  the  Forebrain  and  the  Lateral  Choroid 

Plexuses  in  the  Human  Embryo,"  Journ.  Comp.  Neurol.,  xxvi,  1916. 
C.  R.  Bardeen:  "The  Growth  and  Histogenesis  of  the  Cerebrospinal  Nerves  in 

Mammals,"  Amer.  Jour.  Anat.,  n,  1903. 
S.  R.  Cajal:  "Nouvelles  Observations  sur  revolution  des  neuroblasts  avec  quelques 

remarques  sur  I'hypothese  neurogenetique  de  Hensen-Held,"  Anat.   Anzeiger, 

xxxn,  1908. 
A.  F.  Dixon:  "On  the  Development  of  the  Branches  of  the  Fifth  Cranial  Nerve  in 

Man,"  Scient.  Trans.  Roy.  Dublin  Soc,  Ser.  i,  vi,  1896. 
C.  R.  Essick:  "The  Development  of  the  Nuclei  pontis  and  the  Nucleus  Arcuatus  in 

Man,"  Amer.  Journ.  Anat.,  xin,  1912. 
E.  GiGLio-Tos:  "Sugli  organi  branchiali  e  laterali  di  senso  nell'  uomo  nei  primordi 

del  suo  sviluppo,"  Monit.  Zool.  Ital.,  xin,  1902. 
E.  GiGLio-Tos:  "SulP  origine  embrionale  del  nervo  trigemino  nell'  uomo,"  Anat. 

Anzeiger,  xxi,  1902. 
E.  GiGLio-Tos:  "Sul  primordi  dello  sviluppo  del  nervo  acustico-faciale  nell'  uomo," 

Anat.  Anzeiger,  xxi,  1902. 
K.  Goldstein:   Die  erste   Entwicklung   der   grossen   Hirncommissuren   und   die 

'Verwachsung'  von  Thalamus  und  Striatum"  Archiv  fiir  Anat.  und  Physiol., 

Anat.  Abth.,  1903. 
G.   Groenberg:  "Die    Ontogenese  einer  niederen  Saugergehirns  nach  Untersuch- 

ungen  an  Erinaceus  europaeus,"  Zoolog.  Jahrb.  f.  Anat.  und  Ontogen,  xv,  1901. 
1.  Hardesty:  "On  the  Development  and  Nature  of  the  Neuroglia,"  Amer.  Journ. 

Anat.,  Ill,  1904. 
R.  G.  Harrison:  "  Further  Experiments  on  the  Development  of  Peripheral  Nerves," 

Amer.  Journ.  of  Anat.,  v,  1906. 
W.  His:  "Zur  Geschichte  des  menschlichen  Ruckenmarkes  und  der  Nervenwurzeln," 

Abhandl.  der  k'dnigl.  Sdchsischen  Gesellsch.,  Math.-Physik.  Classe,  xiii,  1886. 
W.  His:  "Zur  Geschichte  des  Gehirns  sowie  der  centralen  und  peripherischen  Ner- 

venbahnen  beim  menschlichen  Embryo,"   Abhandl.   der  Konigl.   Sdchsischen 

Gesellsch.,  Math.-Physik.  Classe,  xiv,  1888. 
W.  His:  "Die  Formentwickelung  des  menschlichen  Vorderhirns  vom  Ende  des 

ersten  bis  zum  Beginn  des  dritten  Monats,"  Abhandl.  der  k'dnigl.  Sdchsischen 

Gesellsch.,  Math.-Physik.  Classe,  xv,  1889. 
W.  His:  "Histogenese  und  Zusammenhang  der  Nervenelemente,"  Archiv  fiir  Anat. 

und  Physiol.,  Anat.  Abth.,  Supplement,  1890. 
W.  His:  "Die  Entwickelung  des  menschlichen  Gehirns  wahrend  der  ersten  Monate," 

Leipzig,  1904. 
W.  His,  Jr. :  " Die  Entwickelung  des  Herznervensystem  bei  Wirbelthieren,"  Abhandl. 

der  konigl.  Sdchsischen  Gesellsch.,  Math.-Physik.  Classe,  xviii,  1893. 
W.  His,  Jr:  "Ueber  die  Entwickelung  des  Bauchsympathicus  beim  Hiihnchen  und 

Menschen,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  Supplement,  1897. 
C.  J.  Herrick:  "The  Cranial  and  First  Spinal  Nerves  of  Menidia:  A  Contribution 
upon  the  Nerve  Components  of  the  Bony  Fishes,"  Journ.  of  Comp.  Neurol., 
DC,  1899. 


LITERATURE  43 1 

C.  J.  Herrick:  "The  Cranial  Nerves  and  Cutaneous  Sense-organs  of  the  North 
American  Siluroid  Fishes,"  Journ.  of  Comp.  Neurol.,  xi,  1901. 

C.  H.  Heuser:  "The  Development  of  the  Cerebral  Ventricles  in  the  Pig,"  Amet^ 
Journ.  AnaL,  xv,  1913. 

F.  Hochstetter:  "Ueber  die  Entwickelung  der  Plexus  chorioidei  der  Seitenkam- 

mern  des  menschlichen  Gehirns,"  Anat.  Anzeiger,  xlv,  1913. 

G.  C.  Huber:  "Four  Lectures  on  the  Sympathetic  Nervous  System,"  Journ.  of 

Comp.  Neurol.,  vii,  1897. 
J.  B.  Johnston:  "The  Nervus  Terminalis  in  Man  and  Mammals,"  Anat.  Record, 

VIII,  1914. 
A.  KuNTz:  "A  Contribution  to  the  Histogenesis  of  the  Sympathetic  System,"  Anat. 

Record,  in,  1909. 
A.  KuNTz:  "The  rdle  of  the  Vagi  in  the  Development  of  the  Sympathetic  Nervous 

System,"  Anat.  Anzeiger,  xxxv,  1909. 
A.  KuNTz:  "The  Development  of  the  Sympathetic  Nervous  System  in  Mammals," 

Journ.  Compar.  Neurol.,  xx,  19 10. 
A.  KuNTz:  "The  Development  of  the  Cranial  Sympathetic  Ganglia  in  the  Pig," 

Journ.  Comp.  Neurol.,  xxiii,  19 13. 
M.  VON Lenhossek:  "Die  Entwickelung  der  Ganglienanlagen  bei  dem  menschlichen 

Embryo,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Ahth.,  1891. 

F.  Marchand:  "Ueber  die  Entwickelung  des  Balkens  im  menschlichen  Gehirn," 

Archiv  fUr  mikrosk.  Anat.,  xxxvu,  iSgi. 

E.  L.  Mellus:  "The  Development  of  the  Cerebral  Cortex,"  Amer.  Journ.  AnaL, 

xrv,  1912. 
V.  VON  Mihalkovicz:  "  Entwickelungsgeschichte  des  Gehirns,"  Leipzig,  1877. 
A.    D.    Onodi:  "Ueber    die   Entwickelung    des    sympathischen    Nervensystems," 

Archiv.  fur  mikrosk.  Anat.,  xxvn,  1886. 
C.  W.  Prentiss:  "The  Development  of  the  Hypoglossal  Ganglia  of  Pig  Embryos," 

Journ.  Comp.  Neurol.,  xx,  1910. 

G.  Retzius:  "Das  Menschenhirn,"  Stockholm,  1896. 

A.  Schaper:  "Die  friihesten  Differenzirungsvorgange  im  Cenlralnervensystem," 

Archiv  fur  Entwicklungsmechanik,  v,  1897. 
G.  L.  Streeter:  "The  Development  of  the  Cranial  and  Spinal  Nerves  in  the 

Occipital  Region  of  the  Human  Embryo,"  Amer.  Journ.  Anat.,  iv,  1904. 
G.  L.  Streeter:  "Factors  involved  in  the  formation  of  theFilum  terminale,"  Amer. 

Journ.  Anat.,  xxv,  19 19. 
O.  Strong:  "The  Cranial  Nerves  of  Amphibia,"  Journ.  of  Morphology,  x,  1895. 

F.  Tilney:  "An  Analysis  of  the  Juxta-neural  Epithelial  portion  of  the  Hypophysis 

Cerebri,  with  an  Embryological  and  Histological  Account  of  a  Hitherto  Unde- 

scribed  Part  of  the  Organ,"  Internal.  Monastschr.  Anat.  and  Physiol.,  xxx,  1914- 
R.  Wlassak:  "Die  Herkunft  des  Myelins,"  Archiv  fUr  Entwicklungsmechanik,  vi, 

1898. 
E.   Zuckerkandl:  "Zur    Entwicklung  des   Balkens,"   Arbeiten  aus  neural.   Inst. 

Wien.  xvii,  1909. 


CHAPTER  XVI 

THE  DEVELOPMENT  OF  THE  ORGANS  OF  SPECIAL 

SENSE 

Like  the  cells  of  the  central  nervous  system,  the  sensory  cells 
are  all  of  ectodermal  origin,  and  in  lower  animals,  such  as  the 
earth-worm,  for  instance,  they  retain  their  original  position  in  the 
ectodermal  epithelium  throughout  life.  In  the  vertebrates,  how- 
ever, the  majority  of  the  sensory  cells  relinquish  their  superficial 
position  and  sink  more  or  less  deeply  into  the  subjacent  tissues, 
being  represented  by  the  posterior  root  ganglion  cells  and  by  the 
sensory  cells  of  the  special  sense-organs,  and  it  is  only  in  the  ol- 
factory organ  that  the  original  condition  is  retained.  Those  cells 
which  have  withdrawn  from  the  surface  receive  stimuli  only 
through  overlying  cells,  and  in  certain  cases  these  transmitting 
cells  are  not  specially  differentiated,  the  terminal  branches  of  the 
sensory  dendrites  ending  among  ordinary  epithelial  cells  or  in 
such  structures  as  the  Pacinian  bodies  or  the  end-bulbs  of  Krause 
situated  beneath  undifferentiated  epithelium.  In  other  cases, 
however,  certain  specially  modified  superficial  cells  serve  to  trans- 
mit the  stimuli  to  the  peripheral  sensory  neurones,  forming  such 
structures  as  the  hair-cells  of  the  auditory  epithelium  or  the 
gustatory  cells  of  the  taste-buds. 

Thus  three  degrees  of  differentiation  of  the  special  sensory  cells 
may  be  recognized  and  a  classification  of  the  sense-organs  may  be 
made  upon  this  basis.  One  organ,  however,  the  eye,  cannot  be 
brought  into  such  a  classification,  since  its  sensory  cells  present 
certain  developmental  pecularities  which  distinguish  them  from 
those  of  all  other  sense-organs.  Embryologically  the  retina  is  a 
portion  of  the  central  nervous  system  and  not  a  peripheral  organ, 
and  hence  it  will  be  convenient  to  arrange  the  other  sense-organs 

432 


THE    OLFACTORY   ORGANS  433 

according  to  the  classification  indicated  and  to  discuss  the  history 
of  the  eye  at  the  close  of  the  chapter. 

The  Development  of  the  Olfactory  Organs. — The  general 
development  of  the  nasal  fossa,  the  epithelium  of  which  contains 
the  olfactory  sense  cells,  has  already  been  described  (pp.  102  and 
285),  as  has  also  the  development  of  the  olfactory  lobes  of  the 
brain  (p.  411) ,  and  there  remain  for  consideration  here  merely  the 
formation  of  the  olfactory  nerve  and  the  development  of  the 
rudimentary  organ  of  Jacobson. 

The  Olfactory  Nerve. — Very  diverse  results  have  been  obtained 
by  various  observers  of  the  development  of  the  olfactory  nerve, 
it  having  been  held  at  different  times  that  it  was  formed  by  the 
outgrowth  of  fibers  from  the  olfactory  lobes  (Marshall),  from 
fibers  which  arise  partly  from  the  olfactory  lobes  and  partly  from 
the  olfactory  epithelium  (Beard),  from  the  cells  of  an  olfactory 
ganglion  originally  derived  from  the  olfactory  epithelium  but  later 
separating  from  it  (His),  and  finally,  that  it  was  composed  of  the 
prolongations  of  certain  cells  situated  and,  for  the  most  part  at 
least,  remaining  permanently  in  the  olfactory  epithelium  (Disse). 
The  most  recent  observations  on  the  structure  of  the  olfactory 
epithelium  and  nerve  indicate  a  greater  amount  of  probability  in 
the  last  result  than  in  the  others,  and  the  description  which 
follows  will  be  based  upon  the  observations  of  His,  modified  in 
conformity  with  the  results  obtained  by  Disse  from  chick  embryos. 

In  human  embryos  of  the  fourth  week  the  cells  lining  the 
upper  part  of  the  olfactory  pits  show  a  distinction  into  ordinary 
epithelial  and  sensory  cells,  the  latter,  when  fully  formed,  being 
elongated  cells  prolonged  peripherally  into  a  short  but  narrow 
process  which  reaches  the  surface  of  the  epithelium  and  proximally 
gives  rise  to  an  axis-cylinder  process  which  extends  up  toward 
and  penetrates  the  tip  of  the  olfactory  lobe  to  come  into  contact 
with  the  dendrites  of  the  first  central  neurones  of  the  olfactory 
tract  (Fig.  258).  These  cells  constitute  a  neuro-epithelium  and  in 
later  stages  of  development  retain  their  epithelial  position  for  the 
most  part,  a  few  of  them,  however,  withdrawing  into  the  sub- 
jacent mesenchyme  and  becoming  bipolar,  their  peripheral  pro- 

28 


434 


THE    OLFACTORY    ORGANS 


longations  ending  freely  among  the  cells  of  the  olfactory  epithelium . 
These  bipolar  cells  resemble  closely  in  form  and  relations  the  cells 
of  the  embryonic  posterior  root  ganglia,  and  thus  form  an  interest- 
ing transition  between  these  and  the  neuro-epithelial  cells. 

The  Organ  of  Jacob  son. — In  embryos  of  three  or  four  months  a 
small   pouch-like   invagination   of   the   epithelium   covering   the 


Fig.  258. — Diagram  Illustrating  the  Relations  of  the  Fibers  of  the  Ol- 
factory Nerve. 
Ep,  Epithelium  of  the  olfactory  pit;  C,  cribiform  plate  of  the  ethmoid,  G,  glomerulus 
of  the  olfactory  bulb;  M,  mitral  cell. — {Van  Gehuchten.) 

lower  anterior  portion  of  the  median  septum  of  the  nose  can  readily 
be  seen.  This  becomes  converted  into  a  slender  pouch,  3  to  5 
mm.  long,  ending  blindly  at  its  posterior  extremity  and  opening 
at  its  other  end  into  the  nasal  cavity.  Its  lining  epithelium  re- 
sembles that  of  the  respiratory  portion  of  the  nasal  cavity,  and 


I 

I 


THE    ORGANS    OF    TASTE  435 

there  is  developed  in  the  connective  tissue  beneath  its  floor  a 
slender  plate  of  cartilage,  distinct  from  that  forming  the  septum 
of  the  nose. 

This  organ,  which  may  apparently  undergo  degeneration  in 
the  adult,  and  in  some  cases  completely  disappears,  appears  to 
be  the  representative  of  what  is  known  as  Jacobson's  organ,  a 
structure  which  reaches  a  much  more  extensive  degree  of  develop- 
ment in  many  of  the  lower  mammals,  and  in  these  contains  in  its 
epithelium  sensory  cells  whose  axis-cylinder  processes  pass  with 
those  of  the  olfactory  sense  cells  to  the  olfactory  bulbs.  In  man, 
however,  it  seems  to  be  a  rudimentary  organ,  and  no  satisfactory 
explanation  of  its  function  has  as  yet  been  advanced. 

The  olfactory  neuro- epithelium,  considered  from  a  comparative 
standpoint,  seems  to  have  been  derived  from  the  system  of  lateral 
line  organs  so  highly  developed  in  the  lower  vertebrates  (Kupffer). 
In  higher  forms  the  system,  which  is  cutaneous  in  character,  has 
disappeared,  except  in  two  regions  where  it  has  become  highly 
specialized.  In  one  of  these  regions  it  has  given  rise  to  the  ol- 
factory sense  cells  and  in  the  other  to  the  sim'ilar  cells  of  the 
auditory  apparatus. 

The  Organs  of  Touch  and  Taste. — ^Little  is  yet  known  con- 
cerning the  development  of  the  various  forms  of  tactile  organs 
which  belong  to  the  second  class  of  sensory  organs  described  above. 

The  Organs  of  Taste. — The  remaining  organs  of  special  sense 
belong  to  the  third  class,  and  of  these  the  organs  of  taste  present 
in  many  respects  the  simplest  condition.  They  are  developed 
principally  in  connection  with  the  vallate  and  foliate  papillae  of 
the  tongue,  and  of  the  former  one  of  the  earliest  observed  stages 
has  been  found  in  embryos  of  9  cm.  in  the  form  of  two  ridges  of 
epidermis,  lying  toward  the  back  part  of  the  tongue  and  inclined 
to  one  another  in  such  a  manner  as  to  form  a  V  with  the  apex  di- 
rected backward.  From  these  ridges  solid  downgrowths  of  epi- 
dermis into  the  subjacent  tissue  occur,  each  downgrowth  having 
the  form  of  a  hollow  truncated  cone  with  its  basal  edge  continuous 
with  the  superficial  epidermis  (Fig.  259,  A).  In  later  stages 
cylindrical  outgrowths  develop  from  the  deeper  edges  of  the  cone, 


436  THE    INTERNAL    EAR 

and  about  the  same  time  clefts  appear  in  the  substance  of  the 
original  downgrowths  (Fig.  259,  B),  and,  uniting  together,  finally 
open  to  the  surface,  forming  a  trench  surrounding  a  papilla  (Fig. 
259,  C).  The  outgrowths,  which  are  at  first  solid,  also  undergo 
an  axial  degeneration  and  become  converted  into  the  glands  of 
Ebner  (b),  which  open  into  the  trench  at  or  near  its  floor.  The 
various  papillae  which  occur  in  the  adult  do  not  develop  simul- 
taneously, but  their  number  increases  with  the  age  of  the  fetus, 
and  there  is,  moreover,  considerable  variation  in  the  time  of  their 
development. 

The  taste-buds  are  formed  by  a  differentiation  of  the  epithe- 
lium which  covers  the  papillae,  and  this  differentiation  appears  to 
stand  in  intimate  relation  with  the  penetration  of  fibers  of  the 
glossopharyngeal   nerve   into    the   papillae.     The   buds   form   at 


Pig.  259. — Diagrams  Representing  the  Development  of  a  Vallate  Papilla. 
a.  Valley  surrounding  the  papilla;  b,  von  Ebner's  gland. — (Graberg.) 

various  places  upon  the  papillae,  and  at  one  period  are  especially 
abundant  upon  their  free  surfaces,  but  in  the  later  weeks  of  in- 
trauterine life  these  surface  buds  undergo  degeneration  and  only 
those  upon  the  sides  of  the  trench  persist,  as  a  rule. 

The  foliate  papillae  do  not  seem  to  be  developed  until  some 
time  after  the  circumvallate,  being  entirely  wanting  in  embryos  of 
four  and  a  half  and  five  months,  although  plainly  recognizable  at 
the  seventh  month. 

The  Development  of  the  Ear.— It  is  customary  to  describe  the 
mammalian  ear  as  consisting  of  three  parts,  known  as  the  inner, 
middle,  and  outer  ears,  and  this  division  is,  to  a  certain  extent  at 
least,  confirmed  by  the  embryonic  development.  The  inner  ear, 
which  is  the  sensory  portion  proper,  is  an  ectodermal  structure, 
which  secondarily  becomes  deeply  seated  in  the  mesodermal  tissue 


THE    INTERNAL   EAR  437 

of  the  head,  while  the  middle  and  outer  ears,  which  provide  the 
apparatus  necessary  for  the  conduction  of  the  sound-waves  to  the 
inner  ear,  are  modified  portions  of  the  anterior  branchial  arches. 
It  will  be  convenient,  accordingly,  in  the  description  of  the  ear,  to 
accept  the  usually  recognized  divisions  and  to  consider  first  of  all 
the  development  of  the  inner  ear,  or,  as  it  is  better  termed,  the 
otocyst. 

The  Development  of  the  Otocyst. — In  an  embryo  of  2.4  mm.  a 
pair  of  pits  occur  upon  the  surface  of  the  body  about  opposite  the 
middle  portion  of  the  hind-brain  (Fig.  260,  A).  The  ectoderm 
lining  the  pits  is  somewhat  thicker  than  is  the  neighboring  ecto- 
derm of  the  surface  of  the  body,  and,  from  analogy  with  what 


A  '^  B 

Fig.  260. — Transverse  Section  Passing  through  the  Otocyst  (o/)  of  Embryos 
OF   {A)   2.4  MM.   AND   {B)   4  MM. — {His.) 

occurs  in  other  vertebrates,  it  seems  probable  that  the  pits  are 
formed  by  the  invagination  of  localized  thickenings  of  the  ecto- 
derm. The  mouth  of  each  pit  gradually  becomes  smaller,  until 
finally  the  invagination  is  converted  into  a  closed  sac  (Fig.  260,  B)^ 
which  separates  from  the  surface  ectoderm  and  becomes  enclosed 
within  the  subjacent  mesoderm.  This  sac  is  the  otocyst,  and  in 
the  stage  just  described,  found  in  embryos  of  4  mm.,  it  has  an  oval 
or  more  or  less  spherical  form.  Soon,  however,  in  embryos  of 
6.9  mm.,  a  prolongation  arises  from  its  dorsal  portion  and  the  sac 
assumes  the  form  shown  in  Fig.  261,  A;  this  prolongation,  which  is 
held  by  some  authors  to  be  the  remains  of  the  stalk  which  origi- 
nally connected  the  otocyst  sac  with  the  surface  ectoderm,  repre- 
sents the  ductus  endolymphaticus ,  and,  increasing  in  length,  it  soon 
becomes   a  strong   club-shaped  process,  projecting  considerably 


438 


THE    INTERNAL   EAR 


beyond  the  remaining  portions  of  the  otocyst  (Fig.  261,  B).  In 
embryos  of  about  10.2  mm.  the  sac  begins  to  show  certain  other 
irregularities  of  shape  (Fig.  261,  B,  sc).  Thus,  about  opposite 
the  point  of  origin  of  the  ductus  endolymphaticus  three  folds  make 
their  appearance,  representing  the  semicircular  ducts,  and  as  they 
increase  in  size  the  opposite  walls  of  the  central  portion  of  each 
fold  come  together,  fuse,  and  finally  become  absorbed,  leaving 


rsc 


Pig.  261. — Reconstruction  of  the  Otocysts  of  Embryos  of  (A)  6.9  mm.  and 

(B)    10.2  MM. 
de.    Endolymphatic    duct;    gc,    ganglion   cochleare;    gg,    ganglion   geniculatum;    gv, 
ganglion  vestibulare;  sc,  lateral  semicircular  duct. — (His,  Jr.) 

the  free  edge  of  the  fold  as  a  crescentic  canal,  at  one  end  of  which 
an  enlargement  appears  to  form  the  ampulla.  The  transforma- 
tion of  the  folds  into  canals  takes  place  somewhat  earlier  in  the 
cases  of  the  two  vertical  than  in  that  of  the  horizontal  duct,  as 
may  be  seen  from  Fig.  262,  which  represents  the  condition  occur- 
ring in  an  embryo  of  13.5  mm. 

A  short  distance  below  the  level  at  which  the  canals  communi- 
cate with  the  remaining  portion  of  the  otocyst  a  constriction  ap- 


THE   INTERNAL   EAR 


439 


pears,  indicating  a  separation  of  the  otocyst  into  a  more  dorsal 
portion  and  a  more  ventral  one.  Later,  the  latter  begins  to  be 
prolonged  into  a  flattened  canal  which,  as  it  elongates,  becomes 
coiled  up^n  itself  and  also  becomes  separated  by  a  constriction 
from  the  remaining  portion  of  the 
otocyst  (Fig.  263).  This  canal  is  the 
ductus  cohlearis  (scala  media  of  the 
cochlea),  and  the  remaining  portion 
of  the  otocyst  subsequently  becomes 
divided  by  a  constriction  into  the 
utriculus,  with  which  the  semicircular 
ducts  are  connected,  and  the  sacculus. 
The  constriction  which  separates  the 
cochlear  duct  from  the  sacculus  be- 
comes the  ductus  reuniens,  while  that 
between  the  utriculus  and  sacculus  is 
converted  into  a  narrow  canal  with 
which  the  ductus  endolymphaticus 
connects,  and  hence  it  is  that,  in  the 
adult,  the  connection  between  these 
two  portions  of  the  otocyst  seems  to 
be  formed  by  the  ductus  dividing 
proximally  into  two  limbs,  one  of 
which  is  connected  with  the  utricle 
and  the  other  with  the  saccule. 

When  first  observed  in  the  human 
embryo  the  auditory  ganglion  is 
closely  associated  with  the  geniculate 
ganglion  of  the  seventh  nerve  (Fig. 
261,  B),  the  two,  usually  spoken  of  as  the  acustico-facialis  ganglion, 
forming  a  mass  of  cells  lying  in  close  contact  with  the  anterior 
wall  of'  the  otocyst.  The  origin  of  the  ganglionic  mass  has  not 
yet  been  traced  in  the  mammalia,  but  it  has  been  observed  that 
in  cow  embryos  the  geniculate  ganglion  is  connected  with  the 
ectoderm  at  the  dorsal  end  of  the  first  branchial  cleft  (Froriep) , 
and  it  may  perhaps  be  regarded  as  one  of  the  epibranchial  placodes 


Pig.  262. —  Reconstruction 
OF  THE  Otocyst  of  an  Embryo 
OF  13.5  MM. 

CO,  Cochlea;  de,  endolymphatic 
duct;  sc,  semicircular  duct. — (His, 
Jr.) 


440 


THE    INTERNAL    EAR 


(see  p.  422),  and  in  the  lower  vertebrates  a  union  of  the  ganglion 
with  a  suprabranchial  placode  has  been  observed  (Kupffer),  this 
union  indicating  the  origin  of  the  auditory  ganglion  from  one  or 
more  of  the  ganglia  of  the  lateral  line  system.  ^ 

At  an  early  stage  in  the  human  embryo  the  auditory  ganglion 
shows  indications  of  a  division  into  two  portions,  a  more  dorsal  one, 
which  represents  the  future  ganglion  vestibulare,  and  a  ventral  one, 
the  ganglion  cochleare.  The  ganglion  cells  become  bipolar,  in 
which  condition  they  remain  throughout  life  never  reaching  the 

de         CO       ASds 


Fig.  263. — Reconstruction  of  the  Otocyst  of  an  Embryo  of  20  mm.,  Front  View. 
cc.  Common  limb  of  superior  and  posterior  semicircular  ducts;  eg,  cochlear 
ganglion;  co,  cochlea;  de,  endolymphatic  duct;  s,  sacculus;  sdl,  sdp,  and  sds,  lateral, 
posterior  and  superior  semicircular  ducts;  u,  utriculus;  vg,  vestibular  ganglion. — 
iStreeter.) 

T-shaped  condition  found  in  most  of  the  other  peripheral  cerebro- 
spinal ganglia.  One  of  the  prolongations  of  each  cell  is  directed 
centrally  to  form  a  fiber  of  the  auditory  nerve,  while  the  other 
penetrates  the  wall  of  the  otocyst  to  enter  into  relations  with 
certain  specially  modified  cells  which  differentiate  from  its  lining 
epithelium. 

In  the  earliest  stages  the  ectodermal  lining  of  the  otocyst  is 
formed  of  similar  columnar  cells,  but  later  over  the  greater  part 


THE   INTERNAL    EAR 


441 


of  the  surface  the  cells  flatten  down,  only  a  few,  aggregated  to- 
gether to  form  patches,  retaining  the  high  columnar  form  and  de^ 
veloping  hair-like  processes  upon  their  free  surfaces.  These  are 
the  sensory  cells  of  the  ear.  In  the  human  ear  there  are  in  all 
six  patches  of  these  sensory  cells,  an  elongated  patch  (crista  ampul- 
laris)  in  the  ampulla  of  each  semicircular  canal,  a  round  patch 
{macula  acustica),  in  the  utriculus  and  another  in  the  sacculus, 
and,  finally,  an  elongated  patch  which  extends  the  entire  length  of 


^■%.;,-^'^m 


n.^:Y      ]       J' 

0       ^    '"     c'' 


Fig.  264. — Section  of  the  Cochlear  Duct  of  a     Rabbit  Embryo  of  55  mm. 
a,  Mesenchyme;  b,  to  e,  epithelium  of  cochlear  duct;  M.t,  membrana  tectoria;  V.s.p, 
vein;  i  to  7,  spiral  organ  of  Corti. — (Baginsky.) 

the  scala  media  of  the  cochlea  and  forms  the  sensory  cells  of  the 
spiral  organ  of  Corti.  The  cells  of  this  last  patch  are  connected 
with  the  fibers  from  the  cochlear  ganglion,  while  those  of  the 
vestibular  ganglion  pass  to  the  cristae  and  maculae. 

In  connection  with  the  spiral  organ  certain  adjacent  cells  also 
retain  their  columnar  form  and  undergo  various  modifications, 
giving  rise  to  a  rather  complicated  structure  whose  development 
has  been  traced  in  the  rabbit  and  bat.  Along  the  whole  length  of 
the  cochlear  duct  the  cells  resting  upon  that  half  of  the  basilar 


442  THE    INTERNAL    EAR 

membrane  which  is  nearest  the  axis  of  the  cochlea,  and  may  be 
termed  the  inner  half,  retain  their  columnar  shape,  forming  two 
ridges  projecting  slightly  into  the  cavity  of  the  scala  (Fig.  264). 
The  cells  of  the  inner  ridge,  much  the  larger  of  the  two,  give  rise  to 
the  memhrana  tectoria,  as  a  cuticular  secretion  to  which  the  cells  of 
the  outer  ridge  also  contribute.  These  latter  are  arranged  in 
six  longitudinal  rows  (Fig.  264,  1-6);  those  of  the  innermost  row 
(i)  develop  hairs  upon  their  free  surfaces   and  form   the  inner 


Ductus  semicirc.  lat. 


Precartilage 


??etlculum 


PiG.  265. — Section  through  the  Lateral  Semicircular  Canal  of  a  Human 
Embryo  of  50  mm. — {Slreeier.) 

hair  cells,  those  of  the  next  two  rows  (2  and  3)  gradually  become 
transformed  on  their  adjacent  surfaces  into  chitinous  substance 
and  form  the  rods  of  Corti,  while  the  three  outer  rows  (4  to  6) 
develop  into  the  outer  hair  cells.  It  is  in  connection  with  the 
hair  cells  that  the  peripheral  prolongations  of  the  cells  of  the 
cochlear  ganglion  terminate,  and  since  these  hair  cells  are  ar- 
ranged in  rows  extending  the  entire  length  of  the  cochlear  duct, 
the  ganglion  also  is  drawn  out  into  a  spiral  following  the  coils  of 
the  cochlea,  and  hence  is  sometimes  termed  the  spiral  ganglion. 


THH   INTERNAL   EAR  443 

While  the  various  changes  described  above  have  been  taking 
place  in  the  otocyst,  the  mesenchyme  surrounding  it  has  also  been 
undergoing  modification.  At  first  this  tissue  undergoes  a  conden- 
sation around  the  otocyst  and  this  condensed  tissue  later  assumes 
the  character  of  pre-cartilage  and  eventually  of  cartilage,  this  rep- 
resenting the  cartilaginous  stage  of  the  petrous  portion  of  the 
temporal  bone.  The  transformation  of  the  mesenchyme  into 
cartilage  takes  place  in  embryos  of  from  25  to  30  mm.  in  length 
and  at  this  stage  the  cartilage  is  in  close  contact  with  the  walls 
of  the  otocyst.  In  later  stages  (Fig.  265),  however,  the  cartilage 
in  the  immediate  neighborhood  of  the  otocyst  undergoes  a  reversal 
of  development,  returning  to  the  precartilage  condition  and  then 
becoming  a  syncytial  reticulum,  and  eventually  the  meshes  of 
the  reticulum  run  together,  form  cavities  of  greater  or  less 
extent  surrounding  the  various  portions  of  the  otocyst  and 
separating  them  from  contact  with  the  surrounding  cartilage. 

The  first  of  these  periotic  space  to  form  makes  its  appearance 
in  the  region  where  the  stapes  is  in  contact  with  the  otic  capsule 
and  it  gradually  extends  so  as  to  enclose  the  utriculus  and  sac- 
culus,  forming  what  is  termed  the  vestibular  perilymphatic 
space.  From  this  the  formation  of  spaces  extends  into  the  reticu- 
lum around  each  of  the  semicircular  canals,  strands  of  the  reticu- 
lum persisting,  however,  and  imperfectly  dividing  the  space 
associated  with  each  canal.  The  reticulum  associated  with  the 
cochlear  duct  becomes  divided  into  two  portions  by  the  duct 
forming  a  broad  attachment  along  its  convex  surface  to  the  peri- 
chondrium of  the  otic  capsule  and  a  narrow  one  along  its  concave 
surface  to  the  edge  of  a  shelf-like  process  of  cartilage  which  later 
ossifies  to  form  the  lamina  spiralis  (Fig.  266).  Above  and  below 
these  lines  of  attachment  spaces  appear  in  the  reticulum,  gradu- 
ally extending  along  the  entire  length  of  the  cochlear  duct  until 
they  communicate  at  its  apex.  The  upper  space  so  formed  com- 
municates proximally  with  the  vestibular  space  and  forms  what 
is  known  as  the  scala  vestibuli,  while  the  lower  one  terminates 
bhndly  at  its  proximal  end,  at  a  point  where  chondrification  and 
ossification  of  the  otic  capsule  have  failed  to  occur,  producing  what 


444  THE   MIDDLE    EAR 

in  the  macerated  skull  appears  to  be  an  opening  in  the  petrous 
bone,  the  fenestra  cochlecB  (rotunda).  The  opening  is  in  reality 
closed  by  a  thin  membrane  which  separates  the  proximal  end 
of  the  lower  space  from  the  tympanic  cavity,  whence  the  space 
is  known  as  the  scala  tympani;  the  scala  media  is  the  cavity  of 
the  cochlear  duct.  A  second  opening,  the  fenestra  vestibuli  (ovalis), 
also  closed  by  membrane  in  which  the  foot  of  the  stapes  is  em- 
bedded, occurs  in  the  osseous  otic  capsule  opposite  the  utriculus 
and  separates  the  vestibular  space  from  the  tympanic  cavity. 


Fig.  266. — Diagrammatic  Transverse  Section  through  a  Coil  of  the  Cochlea 

SHOWING  the  Relation  of  the  Scal/E. 
c,  Organ  of  Corti;  co,  ganglion  cochleare;  Is,  lamina  spiralis;  SM,  cochlear  duct;  ST, 
scala  tympani;  SV,  scala  vestibuli. — (From  Gerlach.) 

The  Development  of  the  Middle  Ear. — The  middle  ear  develops 
from  the  upper  part  of  the  pharyngeal  groove  which  represents 
the  endodermal  portion  of  the  first  branchial  cleft.  This  be- 
comes prolonged  dorsally  and  at  its  dorsal  end  enlarges  to  form 
the  tympanic  cavity,  while  the  narrower  portion  intervening  be- 
tween this  and  the  pharyngeal  cavity  represents  the  tuba  auditiva 
(Eustachian  tube). 

To  correctly  understand  the  development  of  the  tympanic 
cavity  it  is  necessary  to  recall  the  structures  which  form  its  bound- 


THE   MIDDLE   EAR 


445 


aries.  Anteriorly  to  the  upper  end  of  the  first  branchial  pouch 
there  is  the  upper  end  of  the  first  arch,  and  behind  it  the  corre- 
sponding part  of  the  second  arch,  the  two  fusing  together  dorsal 
to  the  tympanic  cavity  and  forming  its  roof.  Internally  the  cavity 
is  bounded  by  the  outer  wall  of  the  cartilaginous  investment  of 
the  otocyst,  while  externally  it  is  separated  from  the  upper  part 
of  the  ectodermal  groove  of  the  first  branchial  cleft  by  the  thin 
membrane  which  forms  the  floor  of  the  groove. 


Pig.  267. — Semi-diagrammatic  View  of  the  Auditory  Ossicles  of  an  Embryo 

OF  Six  Weeks. 
i.  Incus;  J,  jugular  vein;  m,  malleus;  mc,  Meckel's  cartilage;  oc,  capsule  of  otocyst; 
R,  cartilage  of  the  second  branchial  arch;  st,  stapes;   VII,  facial  nerve.— (5te6eri- 
mann.) 


It  has  been  seen  in  an  earlier  chapter  that  the  axial  mesoderm 
of  each  branchial  arch  gives  rise  to  skeletal  structures  and  muscles . 
The  axial  cartilage  of  the  ventral  portion  of  the  first  arch  is  what  is 
known  as  Meckel's  cartilage,  but  in  that  portion  of  the  arch  which 
forms  the  roof  and  anterior  wall  of  the  tympanic  cavity,  the 
cartilage  becomes  constricted  to  form  two  masses  which  later 
ossify  to  form  the  malleus  and  incus  (Fig.  261,  m  and  i),  while  the 
muscular  tissue  of  this  dorsal  portion  of  the  arch  gives  rise  to  the 
tensor  tympani.     Similarly,  in  the  case  of  the  second  arch  there  is 


446  THE   EXTERNAL   EAR 

to  be  found,  dorsal  to  the  extremity  of  the  cartilage  which  forms 
the  styloid  process  of  the  adult,  a  narrow  plate  of  cartilage  which 
forms  an  investment  for  the  facial  nerve  (Fig.  267,  VII),  and  dorsal 
to  this  a  ring  of  cartilage  (st)  which  surrounds  a  small  stapedial 
artery  and  represents  the  stapes. 
^^  Ithasbeenfound  that  in  the  rabbit 

the  mass  of  cells  from  which  the  stapes 
m.     9^^  is   formed  is  at  its  first  appearance 

"'"—"""•.  quite   independent  of   the    second 

^         J  branchial   arch   (Fuchs),   and  it  has 

'  "    ■  ' "  been  held  to  be  a  derivative  of  the 

^.^<  ,        mesenchyme  from  which  the  periotic 

^^^       "x  capsule  is  formed.     In  later  stages, 

'9J^  however,  it  becomes  connected  with 

* ;  ;  /  the  cartilage  of  the  second  branchial 

-..,        7*  /  arch,  as  shown  in  Fig.  267,  and  it  is  a 

^     question    whether    this    connection, 

p...^^ __^  which  is  transitory,  does  not  really 

fi  /'        .--^^         ■•..  indicate   the   phylogenetic    origin  of 

^.;'  **"''■  \ «.       the    ossicle    from    the    second   arch 

^.  ^  I  >^';       cartilage,  its  appearance  as  an  inde- 

;  ...  ,y      ^     pendent  structure  being  a  secondary 

„        ,„     ^  ^  ontoerenetic   phenomenon.     However 

Fig.    268. — Diagrams  Illus-  o  r- 

TRATiNG  THE  MoDE  OF  ExTEN-    that  may  bc,  thc  stapedial  artery  dis- 

siON  OF  THE   Tympanic  Cavity  •        i    4.  ^  a      *-\^^ 

AROUND  THE  Auditory  Ossicles,  appears    m    later    stages    and    the 

M,  Malleus;  m,  spongy  mesen-  stapedius   musclc,   derived  from   the 

t^^^^^J^:^.  musculature  of  the  second  branchial 

The  broken  line  represents  the  ^rch   and  therefore  suppHcd  by  the 

epithelial  lining  of  the  tympanic  i       i  i 

cavity.  facial  nervc,  becomes  attached  to  the 

ossicle. 
The  three  ossicles  at  first  lie  embedded  in  the  mesenchyme 
forming  the  roof  of  the  primitive  tympanic  cavity,  as  does  also  the 
chorda  tympani,  a  branch  of  the  seventh  nerve,  as  it  passes  into 
the  substance  of  the  first  arch  on  the  way  to  its  destination.  The 
mesenchyme  in  which  these  various  structures  are  embedded  is 
rather  voluminous  (Fig.  269),  and  after  the  end  of  the  seventh  month 


THE   EXTERNAL   EAR  447 

becomes  converted  into  a  peculiar  spongy  tissue,  which,  toward  the 
end  of  fetal  life,  gradually  degenerates,  the  tympanic  cavity 
at  the  same  time  expanding  and  wrapping  itself  around  the  ossicles 
and  the  muscles  attached  to  them  (Fig.  268) .  The  bones  and  their 
muscles,  consequently,  while  appearing  in  the  adult  to  traverse  the 
tympanic  cavity,  are  really  completely  enclosed  within  a  layer  of 
epithelium  continuous  with  that  lining  the  wall  of  the  cavity, 
while  the  handle  of  the  malleus  and  the  chorda  tympani  lie  between 
the  epithelium  of  the  outer  wall  of  the  cavity  and  the  fibrous  meso- 
derm which  forms  the  tympanic  membrane. 

The  extension  of  the  tympanic  cavity  does  not,  however,  cease 
with  its  replacement  of  the  degenerated  spongy  mesenchyme,  but 
toward  the  end  of  fetal  life  it  begins  to  invade  the  substance  of  the 
temporal  bone  by  a  process  similar  to  that  which  produces  the 
ethmoidal  cells  and  the  other  osseous  sinuses  in  connection  with 
the  nasal  cavities  (see  p.  178).  This  process  continues  for  some 
years  after  birth  and  results  in  the  formation  in  the  mastoid  por- 
tion of  the  bone  of  the  so-called  mastoid  cells ,  which  communicate 
with  the  tympanic  cavity  and  have  an  epithelial  lining  continuous 
with  that  of  the  cavity. 

The  lower  portion  of  the  diverticulum  from  the  first  pharyngeal 
groove  which  gives  rise  to  the  tympanic  cavity  becomes  con- 
verted into  the  Eustachian  tube.  During  development  the 
lumen  of  the  tube  disappears  for  a  time,  probably  owing  to  a  pro- 
liferation of  its  lining  epithelium,  but  it  is  re-established  before 
birth. 

In  the  account  of  the  development  of  the  ear-bones  above  it  is 
held  that  the  malleus  and  incus  are  derivatives  of  the  first  branchial 
(mandibular)  arch  and  the  stapes  probably  of  the  second.  This  view 
represents  the  general  consensus  of  recent  workers  on  the  difl&cult  ques- 
tion of  the  origin  of  these  bones,  but  it  should  be  mentioned  that  nearly 
all  possible  modes  of  origin  have  been  at  one  time  or  other  suggested. 
The  malleus  has  very  generally  been  accepted  as  coming  from  the  first 
arch,  and  the  same  is  true  of  the  incus,  although  some  earlier  authors 
have  assigned  it  to  the  second  arch.  ■  But  with  regard  to  the  stapes  the 
opinions  have  been  very  varied.  It  has  been  held  to  be  derived  from  the 
first  arch,  from  the  second  arch,  from  neither  one  nor  the  other,  but  from 
I  the  cartilaginous  investment  of  the  otocyst,  or,  finally,  it  has  been  held 


448 


THE   EXTERNAL   EAR 


to  have  a  compound  origin,  its  arch  being  a  product  of  the  second  arch 
while  its  basal  plate  was  a  part  of  the  otocyst  investment. 

The  Development  of  the  Tympanic  Membrane  and  of  the  Outer 
Ear. — Just  as  the  tympanic  cavity  is  formed  from  the  endodermal 
groove  of  the  first  branchial  cleft,  so  the  outer  ear  owes  its  origin 
to  the  ectodermal  groove  of  the  same  cleft  and  to  the  neighboring 


Fig.  269. — Horizontal  Section  Passing  through  the  Dorsal  Wall  of   the 
External  Auditory  Meatus  in  an  Embryo  of  4.5  mm. 
c.  Cochlea;  de,  endolymphatic  duct;  i,  incus;  /5,  transverse  sinus;  w,  malleus;  me, 
meatus  auditorius  externus;  me',  cavity  of  the  meatus;  s,  sacculus;  sc,  lateral  semi- 
circular canal;  sc\  posterior  semicircular  canal;  st,  stapes;  t,  tympanic  cavity;    u, 
utriculus;  7,  facial  nerve. — (Siebenmann.) 


arches.  The  dorsal  and  most  ventral  portions  of  the  groove  flat- 
ten out  and  disappear,  but  the  median  portion  deepens  to  form,  at 
about  the  end  of  the  second  month,  a  funnel-shaped  cavity  which 
corresponds  to  the  outer  portion  of  the  external  auditory  meatus. 


THE    EXTERNAL   EAR 


449 


From  the  inner  end  of  this  a  solid  ingrowth  of  ectoderm  takes 
place,  and  this,  enlarging  at  its  inner  end  to  form  a  disk-like  mass, 
comes  into  relation  with  the  gelatinous  mesoderm  which  surrounds 
the  malleus  and  chorda  tympani.  At  about  the  seventh  month  a 
split  occurs  in  the  disk- like  mass  (Fig.  269),  separating  it  into  an 
outer  and  an  inner  layer,  the  latter  of  which  becomes  the  outer 
epithelium  of  the  tympanic  membrane.    Later,  the  split  extends 


/    ;' 


.   C 


-rp^ 


Fig.  270. — Stages  in  the  Development  of  the  Auricle. 

A,  Embryo  of  ii  mm.;  B,  of  13.6  mm.,  C,  of  15  mm.;  D,  at  the  beginning  of  the  third 

month;  R,  fetus  of  8.5  cm.;  F,  fetus  at  term, — (His.) 


Outward  in  the  substance  of  the  ectodermal  ingrowth  and 
eventually  unites  with  the  funnel-shaped  cavity  to  complete  the 
external  meatus. 

The  tympanic  membrane  is  formed  in  considerable  part  from 
the  substance  of  the  first  branchial  arch,  the  area  in  which  it 
occurs  not  being  primarily  part  of  the  wall  of  the  tympanic  cavity, 

29 


450  THE    EXTERNAL    EAR 

but  being  brought  into  it  secondarily  by  the  expansion  of  the 
cavity.  The  membrane  itself  is  mesodermal,  in  origin  and  is 
lined  on  its  outer  surface  by  an  ectodermal  and  on  the  inner  by 
an  endodermal  epithelium. 

The  auricle  {pinna)  owes  its  origin  to  the  portions  of  the  first 
and  second  arches  which  bound  the  entrance  of  the  external 
meatus.  Upon  the  posterior  edge  of  the  first  arch  there  appear 
about  the  end  of  the  fourth  week  two  transverse  furrows  which 
mark  off  three  tubercles  (Fig.  270,  A^  1-3)  and  on  the  anterior 
edge  of  the  second  arch  a  corresponding  number  of  tubercles  (4-6) 
is  formed,  while,  in  addition,  a  longitudinal  furrow,  running  down 
the  middle  of  the  arch,  marks  off  a  ridge  {c)  lying  posterior  to  the 
tubercles.  From  these  six  tubercles  and  the  ridge  are  developed 
the  various  parts  of  the  auricle,  as  may  be  seen  from  Fig.  270  which 
represents  the  transformation  as  described  by  His.  According  to 
this,  the  most  ventral  tubercle  of  the  first  arch  (i)  gives  rise  to 
the  tragus  J  and  the  middle  one  (5)  of  the  second  arch  furnishes  the 
antitragus.  The  middle  and  dorsal  tubercles  of  the  first  arch  (2 
and  3)  unite  with  the  ridge  (c)  to  produce  the  helix,  while  from 
the  dorsal  tubercle  of  the  second  arch  (4)  is  produced  the  antehelix 
and  from  the  ventral  one  (6)  the  lobule.  More  recent  observations 
however,  seem  to  indicate  that  the  lobule  is  an  accessory  structure 
unrelated  to  the  tubercles  and  that  the  sixth  tubercle  gives  rise 
to  the  antitragus,  while  the  fifth  is  either  included  in  the  anthelix 
or  else  disappears.  It  is  noteworthy  that  up  to  about  the  third 
month  of  development  the  upper  and  posterior  portion  of  the 
helix  is  bent  forward  so  as  to  conceal  the  anthelix  (Fig.  270,  D)\ 
it  is  just  about  a  corresponding  stage  that  the  pointed  form  of 
the  ear  seen  in  the  lower  mammals  makes  its  appearance,  and  it 
is  evident  that,  were  it  not  for  the  forward  bending,  the  human  ear 
would  also  be  assuming  at  this  stage  a  more  or  less  pointed  form. 
Indeed,  there  is  usually  to  be  found  upon  the  incurved  edge  of  the 
helix,  some  distance  below  the  upper  border  of  the  auricle,  a  more 
or  less  distinct  tubercle,  known  as  Darwin's  tubercle,  which  seems 
to  represent  the  point  of  the  typical  mammalian  ear,  and  is,  ac- 
cordingly, the  morphological  apex  of  the  pinna. 


THE   EYE  451 

There  seems  to  be  little  room  for  doubt  that  the  otocyst  belongs 
primarily  to  the  system  of  lateral  line  sense-organs,  but  a  discussion  of 
this  interesting  question  would  necessitate  a  consideration  of  details 
concerning  the  development  of  the  lower  vertebrates  which  would  be 
foreign  to  the  general  plan  of  this  book.  It  may  be  recalled,  however, 
that  the  analysis  of  the  components  of  the  cranial  nerves  described  on 
page  420  refers  the  auditory  nerve  to  the  lateral  line  system. 


0.  * 


^^ 


Pig.  271. — Early  Stages  in  the  Development  of  the  Lens  in  a  Rabbit  Embryo. 

The  nucleated  layer  to  the  left  is  the  ectoderm  and  the  thicker  lens  epithelium, 
beneath  which  is  the  outer  wall  of  the  optic  evagination;  above  and  below  between 
the  two  is  mesenchyme. — (Rabl.) 

The  Development  of  the  Eye. — The  first  indications  of  the 
development  of  the  eye  are  to  be  found  in  a  pair  of  hollow  out- 


452  THE   EYE 

growths  from  the  side  of  the  first  primary  brain  vesicle,  at  a  level 
which  corresponds  to  the  junction  of  the  dorsal  and  ventral  zones. 
Each  evagination  is  directed  at  first  upward  and  backward,  and, 
enlarging  at  its  extremity,  it  soon  shows  a  differentiation  into  a 
terminal  bulb  and  a  stalk  connecting  the  bulb  with  the  brain 
(Fig.  237).  At  an  early  stage  the  bulb  comes  into  apposition  with 
the  ectoderm  of  the  side  of  the  head,  and  this,  over  the  area  of  con- 
tact, becomes  thickened  and  then  depressed  to  form  the  beginning 
of  the  future  lens  (Fig.  271). 


Fig.  272. — Reconstruction  of  the  Brain  of  an  Embryo  of  Pour  Weeks  show- 
ing  THE  Shorioid  Fissure. — (His.) 

As  the  result  of  the  depression  of  the  lens  ectoderm,  the  outer 
wall  of  the  optic  bulb  becomes  pushed  inward  toward  the  inner 
wall,  and  this  invagination  continuing  until  the  two  walls  come 
into  contact,  the  bulb  is  transformed  into  a  double-walled  cup, 
the  optic  cup,  in  the  mouth  of  which  lies  the  lens  (Fig.  273).  The 
cup  is  not  perfect,  however,  since  the  invagination  affects  not  only 
the  optic  bulb,  but  also  extends  medially  on  the  posterior 
surface  of  the  stalk,  forming  upon  this  a  longitudinal  groove  and 
producing  a  defect  of  the  ventral  wall  of  the  cup,  known  as  the 
chorioidal  fissure  (Fig.  272).  The  groove  and  fissure  become  oc- 
cupied by  mesodermal  tissue,  and  in  this,  at  about  the  fifth  week, 
a  blood-vessel  develops  which  traverses  the  cavity  of  the  cup  to 
reach  the  lens  and  is  known  as  the  arteria  hyaloidea. 


THE   EYE 


453 


In  the  meantime  further  changes  have  been  taking  place  in 
the  lens.  The  ectodermal  depression  which  represents  it  gradu- 
ally deepens  to  form  a  cup,  the  lips  of  which  approximate  and 
finally  meet,  so  that  the  cup  is  converted  into  a  vesicle  which 
finally  separates  completely  from  the  ectoderm  (Fig.  273),  much 
in  the  same  way  as  the  otocyst  does.     As  the  lens  vesicle  is  con- 


FiG.  273. — Horizontal  Section  through  the  Bye  of  an  Embryo  Pig  of  7  mm. 
Br,  Diencephalon;  Ec,  ectoderm;  I,  lens;  P,  pigment,  and  R,  retinal  layers  of  the 

retina. 

stricted  off,  the  surrounding  mesodermal  tissue  grows  in  to  form 
a  layer  between  it  and  the  overlying  ectoderm,  and  a  split  appear- 
ing in  the  layer  divides  it  into  an  outer  thicker  portion,  which 
represents  the  cornea,  and  an  inner  thinner  portion,  which  covers 
the  outer  surface  of  the  lens  and  becomes  highly  vascular.  The 
cavity  between  these  two  portions  represents  the  anterior  chamber 
of  the  eye.     The  cavity  of  the  optic  cup  has  also  become  filled 


454  THE    LENS 

by  a  peculiar  tissue  which  represents  the  vitreous  humors  while  the 
mesodermal  tissue  surrounding  the  cup  condenses  to  form  a  strong 
investment  for  it,  which  is  externally  continuous  with  the  cornea, 
and  at  about  the  sixth  week  shows  a  differentiation  into  an  inner 
vascular  layer,  the  chorioid  coat,  and  an  outer  denser  one,  which 
becomes  the  sclerotic  coat. 

The  various  processes  resulting  in  the  formation  of  the  eye, 
which  have  thus  been  rapidly  sketched,  may  now  be  considered  in 
greater  detail. 

The  Development  of  the  Lens. — When  the  lens  vesicle  is  com- 
plete, it  forms  a  more  or  less  spherical  sac  lying  beneath  the  super- 
ficial ectoderm  and  containing  in  its  cavity  a  few  cells,  either 
scattered  or  in  groups  (Fig.  273).  These  cells,  which  have 
wandered  into  the  cavity  of  the  vesicle  from  its  walls,  take  no  part 
in  the  further  development  of  the  lens,  but  early  undergo  complete 
degeneration,  and  the  first  change  which  is  concerned  with  the 
actual  formation  of  the  lens  is  an  increase  in  the  height  of  the  cells 
forming  its  inner  wall  and  a  thinning  out  of  its  outer  wall  (Fig. 
274,  A).  These  changes  continuing,  the  outer  half  ot  the  vesicle 
becomes  converted  into  a  single  layer  of  somewhat  flat  cells  which 
persist  in  the  adult  condition  to  form  the  anterior  epithelium  of  the 
lens,  while  the  cells  of  the  posterior  wall  form  a  marked  projection 
into  the  cavity  of  the  vesicle  and  eventually  completely  obliterate 
it,  coming  into  contact  with  the  inner  surface  of  the  anterior 
epithelium  (Fig.  274  5). 

These  posterior  elongated  cells  form,  then,  the  principal  mass 
of  the  lens,  and  constitute  what  are  known  as  the  lens  fibers.  At 
first  those  situated  at  the  center  of  the  posterior  wall  are  the 
longest,  the  more  peripheral  ones  gradually  diminishing  in  length 
until  at  the  equator  of  the  lens  they  become  continuous  with  and 
pass  into  the  anterior  epithelium.  As  the  lens  increase  in  size, 
however,  the  most  centrally  situated  cells  fail  to  elongate  as 
rapidly  as  the  more  peripheral  ones  and  are  pushed  in  toward  the 
center  of  the  lens,  the  more  peripheral  fibers  meeting  below  them 
along  a  line  passing  across  the  inner  surface  of  the  lens.  The 
disparity  of  growth  continuing,  a  similar  sutural  line  appears 


THE    LENS 


455 


v»M«f?«' 


•••s^/ 


Fig.  274. — Sections  through  the  Lens  (A)  of  Human  Embryo  of  Thirty  to 
Thirty-one  Days  and  {B)  of  Pig  Embryo  of  36  mm. — (Rabl.) 


456 


THE    LENS 


on  the  outer  surface  beneath  the  anterior  epithelium,  and  the 
fibers  become  arranged  in  concentric  layers  around  a  central  core 
composed  of  the  shorter  fibers.  In  the  human  eye  the  line  of 
suture  of  the  peripheral  fibers  becomes  bent  so  as  to  consist  of 
two  limbs  which  meet  at  an  angle,  and  from  the  angle  a  new 


Fig.  275. 


-Posterior  (Inner)  Surface  of  the  Lens  from  an  Adult  showing  thi 
SuTURAL  Lines. — (Rabl.) 


sutural  line  develops  during  embryonic  life,  so  that  the  sutur< 
assumes  the  form  of  a  three-rayed  star.  In  later  life  the  star* 
become  more  complicated,  being  either  six-rayed  or  more  usuall} 
nine-rayed  in  the  adult  condition  (Fig.  275). 

As  early  as  the  second  month  of  development  the  lens  vesicle 
becomes  completely  invested  by  the  mesodermal  tissue  in  whicl 
blood-vessels  are  developed  in  considerable  numbers,  whence  th< 


THE   OPTIC  CUP  457 

investment  is  termed  the  tunica  vasculosa  lentis  (Fig.  283,  tv).  The 
arteries  of  the  tunic  are  in  connection  principally  with  the  hyaloid 
artery  of  the  vitreous  humor  (Fig.  288),  and  consist  of  numerous" 
fine  branches  which  envelop  the  lens  and  terminate  in  loops  almost 
at  the  center  of  its  outer  surface.  This  tunic  undergoes  degenera- 
tion after  the  seventh  month  of  development,  by  which  time  the 
lens  has  completed  its  period  of  most  active  growth,  and,  as  a  rule, 
completely  disappears  before  birth.  Occasionally,  however,  it 
may  persist  to  a  greater  or  less  extent,  the  persistence  of  the  por- 
tion covering  the  outer  surface  of  the  lens,  known  as  the  memhrana 
pupillarisj  causing  the  malformation  known  as  congenital  atresia 
of  the  pupil. 

In  addition  to  the  vascular  tunic,  the  lens  is  surrounded  by  a 
non-cellular  membrane  termed  the  capsule.  The  origin  of  this 
structure  is  still  in  doubt,  some  observers  maintaining  that  it  is 
a  product  of  the  investing  mesoderm,  while  others  hold  it  to  be  a 
product  of  the  lens  epithelium. 

It  is  interesting  from  the  standpoint  of  developmental  mechanics  to 
note  that  W.  H.  Lewis  and  Spemann  have  shown  that,  in  the  Am- 
phibia, contact  of  the  optic  vesicle  with  the  ectoderm  is  necessary  for  the 
formation  of  the  lens,  and,  furthermore,  if  the  vesicle  be  transplanted  to 
other  regions  of  the  body  of  a  larva,  a  lens  will  be  developed  from  the 
ectoderm  with  which  it  is  then  in  contact,  even  in  the  abdominal  region. 

The  Development  of  the  Optic  Cup. — When  the  invagination  of 
the  outer  wall  of  the  optic  bulb  is  completed,  the  margins  of  the 
resulting  cup  are  opposite  the  sides  of  the  lens  vesicle  (Fig.  273), 
but  with  the  enlargement  of  the  lens  and  cup  the  margins  of  the 
latter  gradually  come  to  lie  in  front  of — that  is  to  say,  upon  the 
outer  surface  of — the  lens,  forming  the  boundary  of  the  opening 
known  as  the  pupil.  The  lens,  consequently,  is  brought  to  lie 
within  the  mouth  of  the  optic  cup,  and  that  portion  of  the  latter 
which  covers  the  lens  takes  part  in  the  formation  of  the  iris  and 
the  adjacent  ciliary  body,  while  its  posterior  portion  gives  rise  to 
the  retina. 

The  chorioidal  fissure  normally  disappears  during  the  sixth  or 
seventh  week  of  development  by  a  fusion  of  its  lips,  and  not  until 


458  THE    IRIS   AND    CILIARY  BODY 

this  is  accomplished  does  the  term  cup  truly  describe  the  form 
assumed  by  the  optic  bulb  after  the  invagination  of  its  outer  wall. 
In  certain  cases  the  lips  of  the  fissure  fail  to  unite  perfectly,  pro- 
ducing the  defect  of  the  eye  known  as  colohoma;  this  may  vary  in 
its  extent,  sometimes  affecting  both  the  iris  and  the  retina  and 
forming  what  is  termed  coloboma  iridis,  and  at  others  being 
confined  to  the  retinal  portion  of  the  cup,  in  which  case  it  is  termed 
coloboma  chorioidae. 

Up  to  a  certain  stage  the  differentiation  of  the  two  layers  which 
form  the  optic  cup  proceeds  along  similar  lines,  in  both  the  ciliary 
and  retinal  regions.  The  layer  which .  represents  the  original 
internal  portion  of  the  bulb  does  not  thicken  as  the  cup  increases 
in  size,  and  becomes  also  the  seat  of  a  deposition  of  dark  pigment, 
whence  it  may  be  termed  the  pigment  layer  of  the  cup;  while  th< 
other  layer — that  formed  by  the  invagination  of  the  outer  portioi 
of  the  bulb,  and  which  may  be  termed  the  retinal  layer — remain^ 
much  thicker  (Fig.  273)  and  in  its  proximal  portions  even  increases 
in  thickness.  Later,  however,  the  development  of  the  ciliary  and 
retinal  portions  of  the  retinal  layers  differs,  and  it  will  be  con- 
venient to  consider  first  the  history  of  the  ciliary  portion. 

The  Development  of  the  Iris  and  Ciliary  Body. — The  first  change 
noticeable  in  the  ciliary  portion  of  the  retinal  layer  is  its  thinning 
out,  a  process  which  continues  until  the  layer  consists,  like  th< 
pigment  layer,  of  but  a  single  layer  of  cells  (Fig.  276),  the  transi- 
tion of  which  to  the  thicker  retinal  portion  of  the  layer  is  some- 
what abrupt  and  corresponds  to  what  is  termed  the  ora  serrata  ii 
adult  anatomy.  In  embryos  of  10.2  cm.  the  retinal  layer  through- 
out its  entire  extent  is  readily  distinguishable  from  the  pigment 
layer  by  the  absence  in  it  of  all  pigmentation,  but  in  older  forma 
this  distinction  gradually  diminishes  in  the  iris  region,  the  retinal 
layer  there  acquiring  pigment  and  forming  the  uvea. 

When  the  anterior  chamber  of  the  eye  is  formed  by  the  splitting 
of  the  mesoderm  which  has  grown  in  between  the  superficial  ecto- 
derm and  the  outer  surface  of  the  lens,  the  peripheral  portions  o: 
its  posterior  (inner)  wall  are  in  relation  with  the  ciliary  portion 
of  the  optic  cup  and  give  rise  to  the  stroma  of  the  ciliary  body  anc 


THE    IRIS    AND    CILIARY  BODY 


45Q 


of  the  iris  (Fig.  276),  this  latter  being  continuous  with  the  tunica 
vasculosa  lentis  so  long  as  that  structure  persists  (Fig.  283).  In 
embryos  of  about  14.5  cm.  the  ciliary  portion  of  the  cup  becomes 
thrown  into  radiating  folds  (Fig.  276),  as  if  by  a  too  rapid  growth, 
and  into  the  folds  lamellae  of  mesoderm  project  from  the  stroma. 
These  folds  occur  not  only  throughout  the  region  of  the  ciHary 
body,  but  also  extend  into  the  iris  region,  where,  however,  they 
are  but  temporary  structures,  disappearing  entirely  by  the  end 


:"^^m.  eit««MBi»«»«  wf»#.«ri««k«aj(iM|^^^ 


***i«ii-j 


Pm 


Fig.  276. — Radial  Section  through  the  Iris  of  an  Embryo  of  19  cm. 
AE,  Pigment  layer;  CC,  ciliary  folds;  IE,  retinal  layer;  I.Str,  iris  stroma;  Pm,  pupil- 
lary membrane;  Rs,  marginal  sinus;  Sph,  sphincter  iridis. — (Szili.) 

of  the  fifth  month.     The  folds  in  the  region  of  the  corpus  ciliare 
persist  and  produce  the  ciliary  processes  of  the  adult  eye. 

Embedded  in  the  substance  of  the  iris  stroma  in  the  adult  are 
non-striped  muscle-fibers,  which  constitute  the  sphincter  and  dila- 
tator iridis.  It  has  long  been  supposed  that  these  fibers  were  dif- 
ferentiated from  the  stroma  of  the  iris,  but  recent  observations 
have  shown  that  they  arise  from  the  cells  of  the  pigment  layer  of 
the  optic  cup,  the  sphincter  appearing  near  the  pupillary  border 
(Fig.  276,  Sph)y  while  the  dilatator  is  more  peripheral. 


460  THE   RETINA 

The  Development  of  the  Retina. — Throughout  the  retinal  region 
of  the  cup  the  pigment  layer,  undergoing  the  same  changes  as  in 
the  ciliary  region,  forms  the  pigment  layer  of  the  retina  (Fig.  274, 
p).  The  retinal  layer  increases  in  thickness  and  early  becomes 
differentiated  into  two  strata  (Fig.  273),  a  thicker  one  lying  next 
the  pigment  layer  and  containing  numerous  nuclei,  and  a  thinner 
one  containing  no  nuclei.     The  thinner  layer,  from  its  position  and 


O        000  o^  Oo 


Fig.  277. — Portion  of  a  Transverse  Section  of  the  Retina  of  a  New-born 

Rabbit. 
ch.  Chorioid  coat;  g,  ganglion-cell  layer;  r,  oiiter  layer  of  nuclei;  p,  pigment  layer. — 

(Falchi.) 

structure,  suggests  an  homology  with  the  marginal  velum  of  the 
central  nervous  system,  and  probably  becomes  converted  into  the 
nerve-fiber  layer  of  the  adult  retina,  the  axis-cylinder  processes 
of  the  ganglion  cells  passing  into  it  on  their  way  to  the  optic  nerve. 
The  thicker  layer  similarly  suggests  a  comparison  with  the  mantle 
layer  of  the  cord  and  brain,  and  in  embryos  of  38  mm.  it  be- 
comes differentiated  into  two  secondary  layers  (Fig.  277),  that 
nearest  the  pigment  layer  (/•)  consisting  of  smaller  and  more  deeply 


THE    RETINA 


461 


staining  nuclei,  probably  representing  the  rod  and  cone  and  bi- 
polar cells  of  the  adult  retina,  while  the  inner  layer,  that  nearest 
the  marginal  velum,  has  larger  nuclei  and  is  presumably  composed 
of  the  ganglion  cells. 

Little  is  as  yet  known  concerning  the  further  differentiation  of 
the  nervous  elements  of  the  human  retina,  but  the  history  of  some 
of  them  has  been  traced  in  the  cat,  in  which,  as  in  other  mammals, 


Fig.  278. — Diagram  showing  the  Development  of  the  Retinal  Elements. 
a.  Cone  cell  in  the  unipolar,  and  b,  in  the  bipolar  stage;  c,  rod  cells  in  the  unipolar, 
and  d,  in  the  bipolar  stage;  e,  bipolar  cells;  /  and  i,  amacrine  cells;  g,  horizontal  cells; 
h,  ganglion  cells;  k,  M tiller's  fiber;  /,  external  limiting  membrane. — (Kallius,  after 
Cajal.) 

the  histogenetic  processes  take  place  at  a  relatively  later  period 
than  in  man.  Of  the  histogenesis  of  the  inner  layer  the  informa- 
tion is  rather  scant,  but  it  may  be  stated  that  the  ganglion  cells 
are  the  earliest  of  all  the  elements  of  the  retina  to  become  recogniz- 
able. The  rod  and  cone  cells,  when  first  distinguishable,  are 
unipolar  cells  (Fig.  278,  a  and  c),  their  single  processes  extending 
outward  from  the  cell-bodies  to  the  external  limiting  membrane 
which  bounds  the  outer  surface  of  the  retinal  layer.     Even  at  an 


462  THE    OPTIC    NERVE 

early  stage  the  cone  cells  (a)  are  distinguishable  from  the  rod  cells 
(c)  by  their  more  decided  reaction  to  silver  salts,  and  at  first  both 
kinds  of  cells  are  scattered  throughout  the  thickness  of  the  layer 
from  which  they  arise.  Later,  a  fine  process  grows  out  from  the 
inner  end  of  each  cell,  which  thus  assumes  a  bipolar  form  (Fig. 
278,  b  and  d),  and,  later  still,  the  cells  gradually  migrate  toward 
the  external  limiting  membrane,  beneath  which  they  form  a 
definite  layer  in  the  adult.  In  the  meantime  there  appears  oppo- 
site the  outer  end  of  each  cell  a  rounded  eminence  projecting  from 
the  outer  surface  of  the  external  limiting  membrance  into  the  pig- 
ment layer.  The  eminences  over  the  cone  cells  are  larger  than 
those  over  the  rod  cells,  and  later,  as  both  increase  in  length,  they 
become  recognizable  by  their  shape  as  the  rods  and  cones. 

The  bipolar  cells  are  not  easily  distinguishable  in  the  early 
stages  of  their  differentiation  from  the  other  cells  with  which  they 
are  mingled,  but  it  is  believed  that  they  are  represented  by  cells 
which  are  bipolar  when  the  rod  and  cone  cells  are  still  in  a  unipolar 
condition  (Fig.  278,  e).  If  this  identification  be  correct,  then  it  is 
noteworthy  that  at  first  their  outer  processes  extend  as  far  as 
the  external  limiting  membrane  and  must  later  shorten  or  fail  to 
elongate  until  their  outer  ends  lie  in  what  is  termed  the  outer 
granular  layer  of  the  retina,  where  they  stand  in  relation  to  the 
inner  ends  of  the  rod  and  cone  cell  processes.  Of  the  development 
of  the  amacrine  (/,  i)  and  horizontal  cells  (g)  of  the  retina  little 
is  known.  From  their  position  in  new-born  kittens  it  seems  prob- 
able that  the  former  are  derived  from  cells  of  the  same  layer  as 
the  ganglion  cells,  while  the  horizontal  cells  may  belong  to  the 
outer  layer. 

In  addition  to  the  various  nerve-elements  mentioned  above 
the  retina  also  contains  neuroglial  elements  known  as  Miiller's 
fibers  (Fig.  278,  k),  which  traverse  the  entire  thickness  of  the  retina. 
The  development  of  these  cells  has  not  yet  been  thoroughly  traced, 
but  they  resemble  closely  the  ependymal  cells  observable  in  early 
stages  of  the  spinal  cord. 

The  Development  of  the  Optic  Nerve. — The  observations  on  the 
development  of  the  retina  have  shown  very  clearly  that  the  great 


THE    OPTIC   NERVE 


463 


majority  of  the  fibers  of  the  optic  nerve  are  axis- cylinders  of  the 
ganglion  cells  of  the  retina  and  grow  from  these  cells  along  the 
optic  stalk  toward  the  brain.  Their  embryonic  history  has  been 
traced  most  thoroughly  in  rat  embryos  (Robinson),  and  what 
follows  is  based  upon  what  has  been  observed  in  that  animal. 
The  optic  stalk,  being  an  outgrowth  from  the  brain,  is  at  first 
a  hollow  structure,  its  cavity  communicating  with  that  of  the  third 
ventricle  at  one  end  and  with  that  of  the  optic  bulb  at  the  other. 
When  the  chorioid  fissure  is  developed,  it  extends,  as  has  already 
been  described,  for  some  distance  along 
the  posterior  surface  of  the  stalk  and 
has  lying  in  it  a  portion  of  the  hyaloid 
artery.  Later,  when  the  lips  of  the 
fissure  fuse,  the  artery  becomes  enclosed 
within  the  stalk  to  form  the  arteria 
centralis  retincB  of  the  adult  (Fig.  281). 
By  the  formation  of  the  fissure  the 
original  cavity  of  the  distal  portion  of 

the   stalk   becomes    obHterated,  and    at  ^^^      ^79. -Diagrammatic 

the  same  time  the  ventral  and  posterior  Longitudinal  Section  of  the 
walls  of  the  stalk  are  brought  into  con- 
tinuity with  the  retinal  layer  of  the 
optic  cup,  and  so  opportunity  is  given 
for  the  passage  of  the  axis-cylinders  of 
the  ganglion  cells  along  those  walls  (Fig. 
279).     At  an  early  stage  a  section  of 

the  proximal  portion  of  the  optic  stalk  (Fig.  280,  A)  shows  the 
central  cavity  surrounded  by  a  number  of  nuclei  representing  the 
mantle  layer,  and  surrounding  these  a  non-nucleated  layer,  re- 
sembling the  marginal  velum  and  continuous  distally  with  the 
similar  layer  of  the  retina.  When  the  gamglion  cells  of  the  latter 
begin  to  send  out  their  axis-cylinder  processes,  these  pass  into  the 
retinal  marginal  velum  and  converge  in  this  layer  tow;ard  the 
bottom  of  the  chorioidal  fissure,  so  reaching  the  ventral  wall  of 
the  optic  stalk,  in  the  velum  of  which  they  may  be  distinguished  in 
rat  embryos  of  4  mm.,  and  still  more  clearly  in  those  of  9  mm. 


Optic  Cup  and  Stalk  passing 

THROUGH     the     ChORIOID    FIS- 
SURE. 

Ah,  Hyaloid  artery;  L,  lens; 
On,  fibers  of  the  optic  nerve; 
Os,  optic  stalk;  PI,  pigment 
layer,  and  R,  retinal  layer  of 
the  retina. 


464  tHH- OPTIC   NERVE 

(Fig.  280,  A).  Later,  as  the  fibers  become  more  numerous,  they 
gradually  invade  the  lateral  and  finally  the  dorsal  walls  of  the  stalk 
and,  at  the  same  time,  the  mantle  cells  of  the  stalk  become  more 
scattered  and  assume  the  form  of  connective-tissue  (neurogUa) 
cells,  while  the  original  cavity  of  the  stalk  is  gradually  obHterated 
(Fig.  280,  B).  Finally,  the  stalk  becomes  a  solid  mass  of  nerve- 
fibers,  among  which  the  altered  mantle  cells  are  scattered. 

From  what  has  been  stated  above  it  will  be  seen  that  the  sensory 
cells  of  the  eye  belong  to  a  somewhat  different  category  from  those  of  the 
other  sense-organs.     Embryologically  they  are  a  specialized  portion  of 


/'    .    '..  . 


A  B 

Fig.  280. — Transverse  Sections  through  the  Proximal  Part  of  the  Optic 
Stalk  of  Rat  Embryos  of  (A)  9  mm.  and  {B)   ii  mm. — (Robinson.) 

the  mantle  layer  of  the  medullary  canal,  whereas  in  the  other  organs 
they  are  peripheral  structures  either  representing  or  being  associated 
with  representatives  of  posterior  root  ganglion  cells.  Viewed  from  this 
standpoint,  and  taking  into  consideration  the  fact  that  the  sensory  por- 
tion of  the  retina  is  formed  from  the  invaginated  part  of  the  optic  bulb, 
some  light  is  thrown  upon  the  inverted  arrangement  of  the  retinal  ele- 
ments, the  rods  and  cones  being  directed  away  from  the  source  of  light. 
The  normal  relations  of  the  mantle  layer  and  marginal  velum  are  re- 
tained in  the  retina,  and  the  latter  serving  as  a  conducting  layer  for  the 
axis-cylinders  of  the  mantle  layer  (ganglion)  cells,  the  layer  of  nerve- 
fibers  becomes  interposed  between  the  source  of  light  and  the  sensory 
cells.  Furthermore,  it  may  be  pointed  out  that  if  the  differentiation  of 
the  retina  be  imagined  to  take  place  before  the  closure  of  the  medullary 
canal — a  condition  which  is  indicated  in  some  of  the  lower  vertebrates — 
there  would  be  then  no  inversion  of  the  elements,  this  peculiarity  being 
due  to  the  conversion  of  the  medullary  plate  into  a  tube,  and  more  espe- 


THE   VITREOUS    HUMOR  465 

cially  to  the  fact  that  the  retina  develops  from  the  outer  wall  of  the  optic 
cup.  In  certain  reptiles  in  which  an  eye  is  developed  in  connection  with 
the  epiphysial  outgrowths  of  the  diencephalon,  the  retinal  portion  of  this 
pineal  eye  is  formed  from  the  inner  layer  of  the  bulb,  and  in  this  case 
there  is  no  inversion  of  the  elements. 

A  justification  of  the  exclusion  of  the  optic  nerve  from  the  category 
which  includes  the  other  cranial  nerves  has  now  been  presented.  For  if 
the  retina  be  regarded  as  a  portion  of  the  central  nervous  system,  it  is 
clear  that  the  nerve  is  not  a  nerve  at  all  in  the  strict  sense  of  that  word, 
but  is  a  tract,  confined  throughout  its  entire  extent  within  the  central 
nervous  system  and  comparable  to  such  groups  of  fibers  as  the  direct 
cerebellar  or  fillet  tracts  of  that  system. 

The  Development  of  the  Vitreous  Humor. — It  has  already  been 
pointed  out  (p.  452)  that  a  blood-vessel,  the  hyaloid  artery,  ac- 
companied by  some  mesodermal  tissue  makes  its  way  into  the 


r 

Fig.  281. — Reconstruction  of  a  Portion  of  the  Eye  of  an  Embryo  of  13.8  mm. 
ah.  Hyaloid  artery;  ch,  chorioid  coat;  /,  lens;  r,  retina. — {His.) 

cavity  of  the  optic  cup  through  the  chorioid  fissure.  On  the  closure 
of  the  fissure  the  artery  becomes  enclosed  within  the  optic  stalk 
and  appears  to  penetrate  the  retina,  upon  the  surface  of  which  its 
branches  ramify.  In  the  embryo  the  artery  does  not,  however, 
terminate  in  these  branches  as  it  does  in  the  adult,  but  is  continued 
on  through  the  cavity  of  the  optic  cup  (Fig.  281)  to  reach  the  lens, 
around  which  it  sends  branches  to  form  the  tunica  vasculosa 
lentis. 

According  to  some  authors,  the  formation  of  the  vitreous 
humor  is  closely  associated  with  the  development  of  this  artery,  the 
humor  being  merely  a  transudate  from  it,  while  others  have  main- 
tained that  it  is  a  derivative  of  the  mesoderm  which  accompanies 

30 


466 


THE  VITREOUS    HUMOR 


the  vessel,  and  is  therefore  to  be  regarded  as  a  peculiar  gelatinuous 
form  of  connective  tissue.  More  recently,  however,  renewed 
observations  by  several  authors  have  resulted  in  the  deposition 
of  the  mesoderm  from  the  chief  role  in  the  formation  of  the  vitreous 
and  the  substitution  in  it  of  the  retina.  At  an  early  stage  of 
development  delicate  protoplasmic  processes  may  be  seen  pro- 
jecting from  the  surface  of  the  retinal  layer  into  the  cavity  of  the 


Fig.  282. — Transverse    Section    through   the   Ciliary   Region  of   a   Chick 
Embryo  of  Sixteen  Days. 
ac.  Anterior  chamber  of  the  eye;  c/,  conjunctiva;  co,  cornea;  *',  iris;  Z,  lens;  mc, 
ciliary  muscle;  rl,  retinal  layer  of  optic  cup;  5/,  spaces  of  Pontana;  si,  suspensory 
ligament  of  the  lens;  v,  vitreous  humor. — {^Angelucci.^ 

optic  cup,  these  processes  probably  arising  from  those  cells  which 
will  later  form  the  Miiller's  (neuroglia)  fibers  of  the  retina.  As 
development  proceeds  they  increase  in  length,  forming  a  dense 
and  very  fine  fibrillar  reticulum  traversing  the  space  between  the 
lens  and  the  retina  and  constituting  the  primary  vitreous  humor. 
The  formation  of  the  fibers  is  especially  active  in  the  ciliary  portion 
of  the  retina  and  it  is  probable  that  it  is  from  some  of  the  fibers 
developing  in  this  region  that  the  suspensory  ligament  of  the  lens 


THE  CORNEA  467 

{zonula  Zinnii)  (Fig.  282,  si)  is  formed,  spaces  which  occur  between 
the  fibers  of  the  ligament  enlarging  to  produce  a  cavity  traversed 
by  scattered  fibers  and  known  as  the  canal  of  Petit. 

A  participation  of  similar  protoplasmic  prolongations  from  the 
cells  of  the  lens  in  the  formation  of  the  vitreous  humor  has  been 
maintained  (von  Lenhossek)  and  as  strenuously  denied.  But  it 
is  generally  admitted  that  at  the  time  when  the  hyaloid  artery 
penetrates  the  vitreous  to  form  the  tunica  vasculosa  lentis  it  carries 
with  it  certain  mesodermal  elements,  whose  fate  is  at  present  un- 
certain. It  has  been  held  that  they  take  part  in  the  formation  of 
the  definite  vitreous,  which,  according  to  this  view,  is  of  mixed 
origin,  being  partly  ectodermal  and  partly  mesodermal  (Van 
Pee),  and,  on  the  contrary,  it  has  been  maintained  that  they 
eventually  undergo  complete  degeneration,  the  vitreous  being  of 
purely  ectodermal  origin  (von  Kolliker). 

The  degeneration  of  the  mesodermal  elements  which  the  latter 
view  supposes  is  associated  with  the  degeneration  of  the  hyaloid 
artery.  This  begins  in  human  embryos  in  the  third  month  and  is 
completed  during  the  ninth  month,  the  only  trace  after  birth  of 
the  existence  of  the  vessel  being  a  more  fluid  consistency  of  the 
axis  of  the  vitreous  humor,  this  more  fluid  portion  representing 
the  space  originally  occupied  by  the  artery  and  forming  what  is 
termed  the  hyaloid  canal  {canal  of  Cloquet). 

The  Development  of  the  Outer  Coat  of  the  Eye,  of  the  Cornea,  and 
of  the  Anterior  Chamber. — Soon  after  the  formation  of  the  optic 
bulb  a  condensation  of  the  mesoderm  cells  around  it  occurs,  form- 
ing a  capsule.  Over  the  medial  portions  of  the  optic  cup  the 
further  differentiation  of  this  capsule  is  comparatively  simple, 
resulting  in  the  formation  of  two  layers,  an  inner  vascular  and 
an  outer  denser  and  fibrous,  the  former  becoming  the  chorioid  coat 
of  the  adult  eye  and  the  latter  the  sclera. 

More  laterally,  however,  the  processes  are  more  complicated. 
After  the  lens  has  separated  from  the  surface  ectoderm  a  thin 
layer  of  mesoderm  grows  in  between  the  two  structures  and  later 
gives  place  to  a  layer  of  homogeneous  substance  in  which  a  few 
cells,  more  numerous  laterally  than  at  the  center,  are  embedded. 


468 


THE  ANTERIOR  CHAMBER  OF  THE  EYE 


Still  later  cells  from  the  adjacent  mesenchyme  grow  into  the  layer, 
which  increases  considerably  in  thickness,  and  blood-vessels  also 
grow  into  that  portion  of  it  which  is  in  contact  with  the  outer 
surface  of  the  lens.  At  this  stage  the  interval  between  the  surface 
ectoderm  and  the  lens  is  occupied  by  a  solid  mass  of  mesodermal 
tissue  (Fig.  283,  co  and  tv),  but  as  development  proceeds,  small 
spaces  {ac)  filled  with  fluid  begin  to  appear  toward  the  inner  por- 
tion of  the  mass,  and  these,  increasing  in  number  and  size,  eventu- 

ac 


eC' 


Fig.  283. — Transverse  Section  through  the  Ciliary  Region  of  a  Pig  Embryo 

OF  23  MM. 

ac.  Anterior  chamber  of  the  eye;  co,  cornea;  ec,  ectoderm;  /,  lens;  mc,  ciliary  mus- 
cle; p,  pigment  layer  of  the  optic  cup;  r,  retinal  layer;  tv,  tunica  vasculosa  lentis. 
— {Angelucci.) 


ally  fuse  together  to  form  a  single  cavity  which  divides  the  mass 
into  an  inner  and  an  outer  portion.  The  cavity  is  the  anterior 
chamber  of  the  eye,  and  it  has  served  to  separate  the  cornea  (co) 
from  the  tunica  vasculosa  lentis  (/y),  and,  extending  laterally  in  all 
directions,  it  also  separates  from  the  cornea  the  mesenchyme  which 
rests  upon  the  marginal  portion  of  the  optic  cup  and  constitutes  I 
the  stroma  of  the  iris.  Cells  arrange  themselves  on  the  corneal 
surface  of  the  cavity  to  form  a  continuous  endothelial  layer,  and  . 


THE   EYELIDS  469 

the  mesenchyme  which  forms  the  peripheral  boundary  of  the 
cavity  assumes  a  fibrous  character  and  forms  the  ligamentum 
pedinatum  iridis,  among  the  fibers  of  which  cavities,  known  as' 
the  spaces  of  Fontana  (Fig.  282, 5/),  apppear.  Beyond  the  margins 
of  the  cavity  the  corneal  tissue  is  directly  continuous  with  the 
sclerotic,  beneath  the  margin  of  which  is  a  distinctly  thickened 
portion  of  mesenchyme  resting  upon  the  ciliary  processes  and 
forming  the  stroma  of  the  ciliary  body,  as  well  as  giving  rise  to 
the  muscle  tissue  which  constitutes  the  ciliary  muscle  (Figs.  282 
and  283,  mc). 

The  ectoderm  which  covers  the  outer  surface  of  the  eye  does  not 
proceed  beyond  the  stage  when  it  consists  of  several  layers  of  cells, 
and  never  develops  a  stratum  corneum.  In  the  corneal  region  it 
rests  directly  upon  the  corneal  tissue,  which  is  thickened  slightly 
upon  its  outer  surface  to  form  the  anterior  elastic  lamina;  more 
peripherally,  however,  a  quantity  of  loose  mesodermal  tissue  lies 
between  the  ectoderm  and  the  outer  surface  of  the  sclerotic,  and, 
together  with  the  ectoderm,  forms  the  conjunctiva  (Fig.  282,  cj). 

The  Development  of  the  Accessory  Apparatus  of  the  Eye. — The 
eyelids  make  their  appearance  at  an  early  stage  as  two  folds  of  skin, 
one  a  short  distance  above  and  the  other  below  the  cornea.  The 
center  of  the  folds  is  at  first  occupied  by  indifferent  mesodermal 
tissue,  which  later  becomes  modified  to  form  the  connective 
tissue  of  the  lids  and  the  tarsal  cartilage,  the  muscle  tissue  probably 
secondarily  growing  into  the  lids  as  a  result  of  the  spreading  of  the 
platysma  over  the  face,  the  orbicularis  oculi  apparently  being  a 
derivative  of  that  sheet  of  muscle  tissue. 

At  about  the  beginning  of  the  third  month  the  lids  have  be- 
come sufficiently  large  to  meet  one  another,  whereupon  the  thick- 
ened epithelium  which  has  formed  upon  their  edges  unites  and  the 
lids  fuse  together,  in  which  condition  they  remain  until  shortly 
before  birth.  During  the  stage  of  fusion  the  eyelashes  (Fig.  284,  h) 
develop  at  the  edges  of  the  lids,  having  the  same  developmental 
history  as  ordinary  hairs,  and  from  the  fused  epithelium  of  each 
lid  there  grow  upward  or  downward,  as  the  case  may  be,  into  the 
mesodermic  tissue,  solid  rods  of  ectoderm,  certain  of  which  early 


470 


THE    EYELIDS 


give  off  numerous  short  lateral  processes  and  become  recognizable 
as  the  tarsal  (Meibomian)  glands  (m),  while  others  retain  the  simple 
cylindrical  form  and  represent  the  glands  of  Moll.  When  the 
eyelids  separate,  these  solid  ingrowths  become  hollow  by  a 
breaking  down  of  their  central  cells,  just  as  in  the  sebaceous  and 
sudoriparous  glands  of  the  skin,  the  tarsal  glands  being  really 


Fig.  284. — Section  through  the  Margins  of  the  Fused  Eyelids  in  an  Embryo 

OF  Six  Months. 
h.  Eyelash;  U,  lower  lid;  m,  tarsal  gland;  mu,  muscle  bundle;  ul,  upper  lid. — {Schiveig- 

ger  Seidl.) 


modifications  of  the  former  glands,  while  the  glands  of  Moll  are 
probably  to  be  regarded  as  specialized  sudoriparous  glands. 

A  third  fold  of  skin,  in  addition  to  the  two  which  produce 
the  eyelids,  is  also  developed  in  connection  with  the  eye,  form- 
ing the  plica  semilunaris.  This  is  a  rudimentary  third  eyelid, 
representing  the  nictitating  membrane  which  is  fairly  well 
developed  in  many  of  the  lower  mammals  and  especially  well  in 
birds. 


THE   LACHRYMAL   GLAND  471 

The  lachrymal  gland  is  developed  at  about  the  third  month  as  a 
number  of  branching  outgrowths  of  the  ectoderm  into  the  adjacent 
mesoderm  along  the  outer  part  of  the  line  where  the  epithelium  of 
the  conjunctiva  becomes  continuous  with  that  covering  the  inner 
surface  of  the  upper  eyelid.  As  in  the  other  epidermal  glands,  the 
outgrowths  and  their  branches  are  at  first  solid,  later  becoming 
hollow  by  the  degeneration  of  their  axial  cells. 

The  naso-lachrymal  duct  is  developed  in  connection  with  the 
groove  which,  at  an  early  stage  in  the  development  (Fig.  63), 
extends  from  the  inner  corner  of  the  eye  to  the  olfactory  pit  and  is 


Fig.  285. — Diagram  showing  the  Insertions  of  the  Lachrymal  Ducts  in 
Embryos  of  40  mm.  and  170  mm.,  the  Caruncula  Lacrimalis  being  formed  in 
the  Latter. 

The  eyelids  are  really  fused  at  these  stages  but  have  been  represented  as  separate 
for  the  sake  of  clearness. — {Ask.) 

bounded  posteriorly  by  the  maxillary  process  of  the  first  visceral 
arch.  The  epithelium  lying  in  the  floor  of  this  groove  thickens 
toward  the  beginning  of  the  sixth  week  to  form  a  solid  cord, 
which  sinks  into  the  subjacent  mesoderm.  From  its  upper  end 
two  outgrowths  arise  which  become  connected  with  the  ectoderm 
of  the  edges  of  the  upper  and  lower  lids,  respectively,  and  represent 
the  lachrymal  ducts ^  and,  finally,  the  solid  cord  and  its  outgrowths 
acquire  a  lumen  and  a  connection  with  the  mucous  membrane  of 
the  inferior  meatus  of  the  nasal  cavity. 

The  inferior  duct  connects  with  the  border  of  the  eyelid  some 
distance  lateral  to  the  inner  angle  of  the  eye,  and  between  its  open- 
ing and  the  angle  a  number  of  tarsal  glands  develop.     The  superior 
j  duct,  on  the  other  hand,  opens  at  first  close  to  the  inner  angle  and 


472  LITERATURE 

later  moves  laterally  until  its  opening  is  opposite  that  of  the  infe- 
rior duct.  During  this  change  the  portion  of  the  lower  lid  between 
the  opening  of  the  inferior  duct  and  the  angle  is  drawn  somewhat 
upward,  and,  with  its  glands,  forms  a  small  reddish  nodule,  resting 
upon  the  plica  semilunaris  and  known  as  the  caruncula  lacrimalis 
(Fig.  285). 

LITERATURE 

G.  Alexander:  "Ueber  Entwicklung  und  Bau  des  Pars  inferior  Labyrinthi  der 
hoheren  Saugethiere,"  Denkschr.  kais.  wissench.  Acad.  Wien,  Math.-Naturw. 
Classe,  Lxx,  1901. 

A.  Angelucci:  "Ueber  Entwickelung  und  Bau  des  vorderen  Uvealtractus  der  Verte- 

braten,"  Archir  fur  mikrosk.  Anat.,  xrx,  1881. 
F.   Ask:  "Ueber  Entwickelung    der  Caruneula  lacrimalis  beim  Menschen,  nebst 
Bemerkungen  iiber  die  Entwickelung  der  Tranenrohrchen  und  derMeibom'schen 
Driisen,"  Anatom.  Anzeiger,  xxx,  1907. 

F.  Ask:  "Ueber  die  Entwicklung  der  Lidrander,  der  Tranenkarunkel  und  der  Nick- 

haut  beim  Menschen,  nebst  Bemerkungen  zur  Entwicklung  der  Tranenabfiihr- 
ungswege,"  Anat.  Hefte,  xxxvi,  1908. 

B.  Baginsky:  "Zur  Entwickelung  der  Gehorschnecke,"  Archiv  fiir  mikrosk.  Anat., 

xxvni,  1886. 
W.  M.  Baldwin:  "Die  Entwicklung  der  Fasern  der  zonula  Zinnii  im  Auge  der 

weissen  Maus  nach  der  Geburt,"  Arch,  fur  mikrosk.  Anat.,  lxxx,  191 2. 
E.  A.  Baumgartner:  "The  Development  of  the  serous  glands  (von  Ebner's)  of  the 

vallate  papillae  in  man,"  Amer.  Journ.  Anat.,  xxn,  1917. 
I.  Broman:  "Die  Entwickelungsgeschichte  der  Gehorknochelchen  beim  Menschen," 

Anat.  Hefte,  xi,  1898. 
S.  Ramon  y  Cajal:  "Nouvelles  contributions  a  I'^tude  histologique  de  la  r6tine," 

Journ.  deVAnat.  et  de  la  Physiol.,  xxxii,  1896. 

G.  Cirincione:  "Ueber  den  gegenwartigen  Stand  der  Frage  hinsichtlich  der  Genese 

des  Glaskorpers,"  Arch,  fiir  Augenheilk.,  l,  1904. 
A.  CoNHNo:  "Ueber  Bau  and  Entwicklung  des  Lidrandes  beim  Menschen,"  Arch, 
fiir  Ophthalmol.,  lxvi,  1908. 

A.  CoNTiNo:  "Ueber  die  Entwicklung  der  Karunkel  und  der  plica  semilunaris  beim 

Menschen,"  Arch,  fiir  Ophthalmol,  lxxi,  1909. 
J.  Disse:  "Die  erste  Entwickelung  der  Riechnerven,"  Anat.  Hefte,  rx,  1897. 

B.  Fleischer:  "Die  Entwickelung  der  Tranenrolirchen  bei  den  Saugetiere/*  Archiv 

fiir  Ophthalmol.,  Lxn,  1906. 

H.  FucHs:  "Bemerkungen  iiber  die  Herkunft  und  Entwickelung  der  Gehorknochel- 
chen bei  Kanichen-Embryonen  (nebst  Bemerkungen  iiber  die  Entwickelung 
des  Knorpelskeletes  der  beiden  ersten  Visceralbogen),"  Archiv.  fiir  Anat  und 
Phys.,  Anat.  Abth.,  Supplement,  1905. 

J.  Graberg:  "Beitrage  zur  Genese  des  Geschmacksorgans  der  Menschen,"  Morphol. 
Arbeiten,  vii,  1898. 

J.  A.  Hammar:  "Zur  allgemeinen  Morphologic  der  Schlundspalten  des  Menschen. 


LITERATURE  473 

Zur  Entwickelungsgeschichte  des  Mittelohrraumes,  des  ausseren  Gehorganges 

und  des  Paukenfelles  beim  Menschen,"  Anat.  Anzeiger,  xx,  1901. 
J.  A.  Hammar:  "Studien  iiber  Entwicklung  des  Vorderdarms  und  einiger  angrenz~ 

ender  Organe,"  Arch,  fur  mikrosk.  Anat.,  lix,  1902. 
C.   Heerfordt:  "Studien  iiber  den  Muse,  dilatator  pupillse  sammt  Angabe  von 

gemeinschaftlicher  Kennzeichen  einiger  Falle  epithelialer  Musculatur,"   Anat, 

Hefte,  XIV. 
J.  Hegetschweiler :  "Die  embryologische  Entwickelung  des  Steigbugels,"  Archiv 

fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1898. 

F.  Hochstetter:  "Ueber  die  Bildung  der  primitiven  Clioanen  beim  Menschen," 

Verhandl.  Anat.  Gesellsch.,  vi,  1892. 
W.    His,   Jr:  "Die   Entwickelungsgeschichte   des   Acustico-Facialisgebietes   beim 

Menschen,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  Supplement,  1897. 
A.  VON  Kolliker:  "Die  Entwicklung  und  Bedeutung  des  Glaskorpers,"  Zeitschr. 

fiir  wissensch.  Zoolog.,  lxxvi,  1904. 
P.  Lang:  Zur  Entwicklung  des  Tranenausfiihrsappa rates  beim  Menschen,"   Anat. 

Anzeiger,^'  xxxviu,  191 1. 

G.  Leboucq:  "Contribution  a  I'^tude  de  I'histog^nSse  de  la  retine  che^  les  mam- 

miferes,"  Arch.  Anat.,  Microsc,  x,  1909. 
J.  Ma  WAS  and  A.  Magitot:  "Etude  sur  le  developpement  du  corps  vitre  et  de  la 

zonule  chez  I'homme,"  Arch.  d'Anat.  Microsc,  xrv,  1912. 
V.  VON  Mihalkovicz:  "Nasenhohle  und  Jacobsonsches  Organ.     Eine  morpholog- 

ische  Studie."     AncU.  Hefte,  xi,  1898. 
J.  L.  Paulet:  "Contribution  a  r6tude  de  I'organe  de  Jacobson  chez  I'embryon 

humain,"  Bibliogr.  Anat.,  xvii,  1907. 
P.  VAN  Pee:  "Recherches  sur  I'origine  du  corps  vilre,"  Archiv  de  Biol.,  XK,  1902. 
C.  W.  Prentiss:  "On  the  Development  of  the  membrana  tectoria  with  reference 

to  its  structure  and  attachments,"  Amer.  Journ.  Anal.,  xiv,  1913. 
C.  Rabl:  "Ueber  den  Bau  und  Entwickelung  der  Linse,"  Zeitschrift  fur  wissensch. 

Zoologie,  Lxn  and  lxv,  1889;  lxviii,  1899. 
A.  Robinson:  "On  the  Formation  and  Structure  of  the  Optic  Nerve  and  Its  Relation 

to  the  Optic  Stalk,"  Journal  of  Anat.  and  Physiol.,  xxx,  1896. 
G.  Speciale-Cirincione:  "Ueber  die  Entwicklung  der   Tranendruse  beim  Men- 
schen," Arch,  fur  Ophthalmol.,  lxix,  1908. 
J.  P.  Schaeffer:  "The  Genesis  and  Development  of  the  Nasolachrymal  Passages  in 

Man,"  Amer.  Journ.  Anat.,  xiii,  191 2. 
U.  Seefelder:  "Beitrage  zur  Entwicklung  des  menschlichen  Auges,"  Anat.  Hefte, 

XLViii,  1 913. 
G.  L.  Streeter    "On  the  Development  of  the  Membranous  Labyrinth  and  the 

Acoustic  and  Facial  Nerves  in  the  Human  Embryo,"  Amer.  Journ.  of  Anat., 

VI,  1907. 
G.  L.  Streeter:  "The  histogenesis  and  growth  of  the  otic  capsule  and  its  contained 

periotic  tissue-spaces  in  the  human  embryo,"  Publications  Carnegie  Inst.,  No. 

227,  Contrib.  to  Embryol.  xx,  1919. 
N.  VAN  dER  Stricht:  "L'histogenese  des  parties  constituantes  du  neuro6pith61ium 

acoustique,  des  taches  et  des  cretes  acoustiques  et  de  I'organe  de  Corti,"  Arch. 

de  Biol.,  xxin,  1908. 


474  LITERATURE 

A.  SziLi:  "Zur  Anatomic  und  Entwickelungsgeschichte  der  hinteren  Irisschichten, 

mit  besonderer  Berucksichtigung  des  Musculus  sphincter  iridis  des  Menschen," 

Anat.  Anzeiger,'KX,  1901. 
A.  SziLi :  "  Ueber  das  Entstehen  eines  fibrillares  Stiitzgewebes  im  Embryo  und  dessen 

Verbal tnis  zur  Glaskorperfrage,"  Anat.  Hefte,  xxxv,  1908. 
F.  Tuckerman:  "On  the  Development  of  the  Taste  Organs  in  Man,"  Journal  of 

Anat.  andFhysioL,-xxiv,  1889. 
R.  Versari:  "Ueber  die  Entwicklung  der  Blutgefasse  des  menschlichen  Auges," 

Anat.  Anzeigefy  xxxv,  1909. 


CHAPTER  XVII 
POST-NATAL  DEVELOPMENT 

In  the  preceding  pages  attention  has  been  directed  principally 
to  the  changes  which  take  place  in  the  various  organs  during  the 
period  before  birth,  for,  with  a  few  exceptions,  notably  that  of 
the  liver,  the  general  form  and  histological  peculiarities  of  the 
various  organs  are  acquired  before  that  epoch.  Development 
does  not,  however,  cease  with  birth,  and  a  few  statements  regard- 
ing the  changes  which  take  place  in  the  interval  between  birth 
and  maturity  will  not  be  out  of  place  in  a  work  of  this  kind. 

The  conditions  which  obtain  during  embryonic  life  are  so 
different  from  those  to  which  the  body  must  later  adapt  itself,  that 
arrangements,  such  as  those  connected  with  the  placental  circula- 
tion, which  are  of  fundamental  importance  during  the  life  in 
utero,  become  of  little  or  no  use,  while  the  relative  importance 
of  others  is  greatly  diminished,  and  these  changes  react  more  or 
less  profoundly  on  all  parts  of  the  body.  Hence,  although  the 
post-natal  development  consists  chiefly  in  the  growth  of  the  struc- 
tures formed  during  earlier  stages,  yet  the  growth  is  not  equally 
rapid  in  all  parts,  and  indeed  in  some  organs  there  may  even  be 
a  relative  decrease  in  size.  That  this  is  true  can  be  seen  from  the 
annexed  figure  (Fig.  286),  which  represents  the  body  of  a  child 
and  that  of  an  adult  man  drawn  as  of  the  same  height.  The 
greater  relative  size  of  the  head  and  upper  part  of  the  body  in  the 
child  is  very  marked,  and  the  central  point  of  the  height  of  the 
child  is  situated  at  about  the  level  of  the  umbilicus,  while  in  the 
man  it  is  at  the  symphysis  pubis. 

That  there  is  a  distinct  change  in  the  geometric  form  of  the 
body  during  growth  is  also  well  shown  by  the  following  considera- 
tion (Thoma).  Taking  the  average  height  of  a  new-born  male  as 
500  mm.,  and  that  of  a  man  of  thirty  years  of  age  as  1686  mm.,  the 

475 


476 


POST-NATAL   DEVELOPMENT 


height  of  the  body  will  have  increased  from  birth  to  adolescence 

1686 

~- —  =  3-37  times.     The  child  will  weigh  3.1  kilos  and  the  man 

66.1  kilos,  and  if  the  specific  gravity  of  the  body  with  the  included 
gases  be  taken  in  the  one  case  as  0.90  and  in  the  other  as  0.93 
then  the  volume  of  the  child's  body  will  be  3.44  liters  and  that  of 

71.08 


the  man's  71.08  liters,  and  the  increase  in  volume  will  be 


3-44 


Fig.  286. — Child  and  Man  Drawn  as  of  the  Same  Height. — (Langer,  from 
the  "Growth  of  the  Brain,"  Contemporary  Science  Series  by  permission  of  Charles 
Scribner's  Sons.) 

20.66.  If  the  increase  in  volume  had  taken  place  without  any 
alteration  in  the  geometric  form  of  the  body,  it  should  be  equal 
to  the  cube  of  the  increase  in  height;  this,  however,  is  3.37^  = 
38.27,  a  number  well-nigh  twice  as  large  as  the  actual  increase. 
But  in  addition  to  these  changes,  which  are  largely  dependent 
upon  differences  in  the  supply  of  nutrition,  there  are  others  associ- 
ated with  alterations  in  the  general  metabolism  of  the  body.  Up 
to  adult  life  the  constructive  metabolism  or  anabolism  is  in  excess 


POST-NATAL   DEVELOPMENT 


477 


of  the  destructive  metabolism  or  katabolism,  but  the  amount  of 
the  excess  is  much  greater  during  the  earlier  periods  of  develop-- 
ment  and  gradually  diminishes  as  the  adult  condition  is  ap- 
proached. That  this  is  true  during  intrauterine  life  is  shown  by 
the  following  figures,  compiled  by  Donaldson: 


Age  in  Weeks 

Weight  in  Grams 

Age  in  Weeks 

Weight  in  Grams 

o  (ovum) 

0.0006 

24 

63s 

4 

— 

28 

1,220 

8 

4.0 

32 

1,700 

12 

20.0 

36 

2,240 

i6 

120.0 

40  (birth) 

3,250 

20 

.       285.0 

From  this  table  it  may  be  seen  that  the  embryo  of  eight  weeks 
is  six  thousand  six  hundred  and  sixty-seven  times  as  heavy  as  the 
ovum  from  which  it  started,  and  if  the  increase  of  growth  for  each 
of  the  succeeding  periods  of  four  weeks  be  represented  as  percent- 
ages, it  will  be  seen  that  the  rate  of  increase  undergoes  a  rapid 
diminution  after  the  sixteenth  week,  and  from  that  on  diminishes 
gradually  but  less  rapidly,  the  figures  being  as  follows : 


Periods  of  Weeks 

Percentage  Increase 

Periods  of  Weeks 

Percentage  Increase 

8-12 
12-16 
16-20 
20-24 

400 
500 

'        137 
123 

24-28 
28-32 
32-36 
36-40 

92 

39 
32 
45 

That  the  same  is  true  in  a  general  way  of  the  growth  after  birth 
may  be  seen  from  the  following  table,  representing  the  average 
weight  of  the  body  in  English  males  at  different  years  from  birth 
up  to  twenty- three  (Roberts),  and  also  the  percentage  rate  of 


mcrease. 


Certain  interesting  peculiarities  in  post-natal  growth  become 
apparent  from  an  examination  of  this  table.  For  while  there  is  a 
general  diminution  in  the  rate  of  growth,  yet  there  are  marked 


478 


POST-NATAL   DEVELOPMENT 


Year 

Number  of  Cases 

Weight  in 
Kilograms 

Percentage 
Increase 

O 

451 

3.2 

I 

— 

(10.8) 

(238) 

2 

2 

14.7* 

(36)* 

3 

41 

154 

4.8* 

4 

I02 

16.9 

9-7 

5 

193 

18. 1 

71 

6 

224 

20.1 

II. 0 

7 

246 

22.6 

12.4 

8 

820 

24.9 

10.2 

9 

1,425 

27.4 

10. 0 

lO 

1,464 

30.6 

II. 5 

II 

1,599 

32.6 

6.5 

12 

1,786 

34.9 

7.0 

13 

2,443 

37.6 

7-7 

■     14 

2,952 

41.7 

10.9 

IS 

3,118 

46.6 

II. 7 

i6 

2,235 

53.9 

157 

17 

2,496 

59.3 

lO.O 

i8 

2,150 

62.2 

4-9 

19 

1,438 

63.4 

1.9 

20 

851 

64.9 

2.5 

21 

738 

65.7 

1.2 

22 

542 

67.0 

1.9 

23 

551 

67.0 

0.0 

irregularities,  the  most  noticeable  being  (i)  a  rather  marked  dimi- 
nution during  the  eleventh  and  twelfth  years,  followed  by  (2)  a 
rapid  acceleration  which  reaches  its  maximum  at  about  the  six- 
teenth year  and  then  very  rapidly  diminishes.  These  irregulari- 
ties may  be  more  clearly  seen  from  the  charts  on  p.  479,  which 
represent  the  curves  obtained  by  plotting  the  annual  increase  of 
weight  in  boys  (Chart  I)  and  girls  (Chart  II).  The  diminution  and 
acceleration  of  growth  referred  to  above  are  clearly  observable  and 

*  From  a  comparison  with  other  similar  tables  there  is  little  doubt  but  that  the 
weight  given  above  for  the  second  year  is  too  high  to  be  accepted  as  a  good  average. 
Consequently  the  percentage  increase  for  the  second  year  is  too  high  and  that  for  the 
third  year  too  low. 

It  may  be  mentioned  that  the  weights  in  the  original  table  are  expressed  in  pounds 
avoirdupois  and  have  been  here  converted  into  kilograms,  and  further  the  figures 
representing  the  percentag  increase  have  been  added. 


POST-NATAL    DEVELOPMENT 


479 


it  is  interesting  to  note  that  they  occur  at  earlier  periods  in  girls 
than  in  boys,  the  diminution  occurring  in  girls  at  the  eighth  and 
ninth  years  and  the  acceleration  reaching  its  maximum  at  the 
thirteenth  year. 

Considering,  now,  merely  the  general  diminution  in  the  rate  of 
growth  which  occurs  from  birth  to  adult  life,  it  becomes  interesting 


I 

Am    "x     ii     -^     i      .      ^'  .      9     \>    i^    12    x3    u    a    X    17    m 

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Fig.  287. — Curves  Showing  the  Annual  Increase  in  Weight  in  (I)  Boys  and 

(II)  Girls. 
The  faint  line  represents  the  curve  from  British  statistics,  the  dotted  line  that 
from  American  (Bowditch),  and  the  heavy  line  the  average  of  the  two.     Before  the 
f  ixth  year  the  data  are  unreliable. — {Stephenson.) 

to  note  to  what  extent  the  organs  which  are  more  immediately 
associated  with  the  metabolic  activities  of  the  body  undergo  a  rela- 
tive reduction  in  weight.  The  most  important  of  these  organs  is 
undoubtedly  the  liver,  but  with  it  there  must  also  be  considered  the 
thyreoid  and  thymus  glands,  and  probably  the  suprarenal  bodies. 


48o 


POST-NATAL   DEVELOPMENT 


In  all  these  organs  there  is  a  marked  diminution  in  size  as  com- 
pared with  the  weight  of  the  body,  as  will  be  seen  from  the  follow- 
ing table  (H.  Vierordt),  which  also  includes  data  regarding  other 
organs  in  which  a  marked  relative  diminution,  not  in  all  cases 
readily  explainable,  occurs. 


ABSOLUTE  WEIGHT 
New-born  and  a 

IN  GRAMS 
\dult 

Liver 

Thy- 
reoid 

Thy- 
mus 

Suprarenal 
Bodies 

Spleen 

Heart 

Kidney 

B-in          'goTd' 

141. 7 
1,819.0 

4.85 
33.8 

8.15 
26.9 

7.05 
7-4 

10.6 
163.0 

23.6 
300.6 

233 
305.9 

381.0 

1,430.9 

1 

5-5 
3915 

PERCENTAGE  WEIGHT  OF  ENTIRE  BODY 

New-born  and  Adult 


Heart 

Kidney 

Brain 

Liver 

Thy- 
reoid 

l^s           'XSS°'       Spleen 

Spinal 
Cord 

4.57          0.16 
2.57          0.05 

0.26 
0.04 

0.23 

O.OI 

0.34      0.76 
0.25       0.46 

0.75 
0.46 

12.29 
2.16 

0.18 
0.06 

Recent  observations  by  Hammar  render  necessary  some  modifica- 
tion of  the  figures  given  for  the  thymus  in  the  above  table.  He  finds 
the  average  weight  of  the  gland  at  birth  to  be  13.26  grams,  and  that 
the  weight  increases  up  to  puberty,  averaging  37.52  grams  between  the 
ages  of  II  and  15.  After  that  period  it  gradually  diminishes,  falling  to 
16.27  grams  between  36  and  45,  and  to  6.0  grams  between  66  and  75. 
Expressed  in  percentage  of  the  body  weight  this  gives  a  value  in  the 
new-born  of  0.42  and  in  an  individual  of  50  years  of  0.02,  a  difference 
much  more  striking  than  that  shown  in  Vierordt's  table. 

It  must  be  mentioned,  however,  that  the  gland  is  subject  to  much 
individual  variation,  being  largely  influenced  by  nutritive  conditions. 

The  remaining  organs,  not  included  in  the  tables  given  above, 
when  compared  with  the  weight  of  the  body,  either  show  an  in- 
crease or  remain  practically  the  same. 


POST-NATAL   DEVELOPMENT 


481 


ABSOLUTE  WEIGHT  IN  GRAMS 
New-born  and  Adult 


Skin  and  Sub- 
cutaneous  Tissues 

Skeleton 

Musculature 

Stomach  and 
Intestines 

Pancreas 

Lungs 

611.75 
11,765.0 

425.5 
11,575.0 

776.5 
28,732.0 

65 
1,364 

3-5 
97.6 

54-1 
994.9 

PERCENTAGE  OF  BODY  WEIGHT 
New-born  and  Adult 

Skin  and  Sub- 
cutaneous Tissues 

Skeleton 

Musculature 

Stomach  and 
Intestines 

Pancreas 

Lungs 

19-73 

17.77 

13.7 
17.48 

25.05 
43.40 

2.1 
2.06 

0.  II 
0.15 

1.75 
1.50 

From  this  table  it  will  be  seen  that  the  greatest  increment  of 
weight  is  that  furnished  by  the  muscles,  the  percentage  weight  of 
which  is  one  and  three-fourths  times  as  great  in  the  adult  as  in  the 
child.  The  difference  does  not,  however,  depend  upon  the  differ- 
entiation of  additional  muscles;  there  are  just  as  many  muscles  in 
the  new-born  child  as  in  the  adult,  and  the  increase  is  due  merely  to 
an  enlargement  of  organs  already  present.  The  percentage  weight 
of  the  digestive  tract,  pancreas,  and  lungs  remains  practically  the 
same,  while  in  the  case  of  the  skeleton  there  is  an  appreciable  in- 
crease, and  in  that  of  the  skin  and  subcutaneous  tissue  a  slight 
diminution.  The  latter  is  readily  understood  when  it  is  remem- 
bered that  the  area  of  the  skin,  granting  that  the  geometric  form 
of  the  body  remains  the  same,  would  increase  as  the  square  of  the 
length,  while  the  mass  of  the  body  would  increase  as  the  cube,  and 
hence  in  comparing  weights  the  skin  might  be  expected  to  show  a 
diminution  even  greater  than  that  shown  in  the  table. 

The  increase  in  the  weight  of  the  skeleton  is  due  to  a  certain 
extent  to  growth,  but  chiefly  to  a  completion  of  the  ossification  of 
the  cartilage  largely  present  at  birth.  A  comparison  of  the 
weights  of  this  system  of  organs  does  not,  therefore,  give  evidence 
of  the  many  changes  of  form  which  may  be  perceived  in  it  during 


31 


482 


POST-NATAL   DEVELOPMENT 


the  period  under  consideration,  and  attention  may  be  drawn  to 
some  of  the  more  important  of  these  changes. 

In  the  spinal  column  one  of  the  most  noticeable  pecularities 
observable  in  the  new-born  child  is  the  absence  of  the  curves  so 
characteristic  of  the  adult.  These  curves  are  due  partly  to  the 
weight  of  the  body,  transmitted  through  the  spinal  column  to  the 
hip-joint  in  the  erect  position,  and  partly  to  the  action  of  the  mus- 


FiG.  288. 


-Longitudinal  Section  through  the  Sacrum  of  a  New-born  Female 
Child. — (Fehling.) 


cles,  and  it  is  not  until  the  erect  position  is  habitually  assumed  and 
the  musculature  gains  in  development  that  the  curvatures  become 
pronounced.  Even  the  curve  of  the  sacrum,  so  marked  in  the 
adult,  is  but  slight  in  the  new-born  child,  as  may  be  seen  from  Fig. 
285,  in  which  the  ventral  surfaces  of  the  first  and  second  sacral 
vertebrae  look  more  ventrally  than  posteriorly,  so  that  there  is  no 
distinct  promontory. 


POST-NATAL    DEVELOPMENT 


483 


But,  in  addition  to  the  appearance  of  the  curvatures,  other 
changes  also  occur  after  birth,  the  entire  column  becoming  much_ 
more  slender  and  the  proportions  of  the  lumbar  and  sacral  vertebrae 
becoming  quite  different,  as  may  be  seen  from  the  following  table 

(Aeby): 


LENGTHS  OF  THE  VERTEBRAL  REGIONS  EXPRESSED  AS  PERCENT- 
AGES OF  THE  ENTIRE  COLUMN 

Age 

Cervical 

Thoracic 

Lumbar 

New-born  child 

25.6 

23.3 
20.3 

19.7 
22.  I 

47-5 
46.7 
45.6 
47.2 
46.6 

26.8 

Male  2  years 

300 
34.2 
33.1 
31.6 

Male  5  years 

Male  1 1  years 

Male  adult 

The  cervical  region  diminishes  in  length,  while  the  lumbar 
gains,  the  thoracic  remaining  approximately  the  same.  It  may  be 
noticed,  furthermore,  that  the  difference  between  the  two  variable 
regions  is  greater  during  youth  than  in  the  adult,  a  condition  possi- 
bly associated  with  the  general  more  rapid  development  of  the 
lower  portion  of  the  body  made  necessary  by  its  imperfect  develop- 
ment during  fetal  life.  The  difference  is  due  to  changes  in  the 
vertebrae,  the  intervertebral  disks  retaining  approximately  the 
same  relative  thickness  throughout  the  period  under  consideration. 

The  form  of  the  thorax  also  alters,  for  whereas  in  the  adult  it  is 
barrel-shaped,  narrower  at  both  top  and  bottom  than  in  the 
middle,  in  the  new-born  child  it  is  rather  conical,  the  base  of  the 
cone  being  below.  The  difference  depends  upon  slight  differences 
in  the  form  and  articulations  of  the  ribs,  these  being  more  horizon- 
tal in  the  child  and  the  opening  of  the  thorax  directed  more  directly 
upward  than  in  the  adult. 

As  regards  the  skull,  the  processes  of  growth  are  very  compli- 
cated. Cranium  and  brain  react  on  one  another,  and  hence,  in 
harmony  with  the  relatively  enormous  size  of  the  brain  at  birth, 
the  cranial  cavity  has  a  relatively  greater  volume  in  the  child  than 
in  the  adult.  The  fact  that  the  entire  roof  and  a  considerable  part 
of  the  sides  of  the  skull  are  formed  of  membrance  bones  which,  at 


484  POST-NATAL   DEVELOPMENT 

birth,  are  not  in  sutural  contact  with  one  another  throughout, 
gives  opportunity  for  considerable  modifications,  and,  furthermore, 
the  base  of  the  skull  at  the  early  stage  still  contains  a  considerable 
amount  of  unossified  cartilage.  Without  entering  into  minute  de- 
tails, it  may  be  stated  that  the  principal  general  changes  which  the 
skull  undergoes  in  its  post-natal  development  are  (i)  a  relative 
elongation  of  its  anterior  portion  and  (2)  an  increase  in  the  relative 
height  of  the  maxillae  . 

If  a  line  be  drawn  between  the  central  points  of  the  occipital 
condyles,  it  will  divide  the  base  ot  the  skull  into  two  portions, 


Fig.  289. — Skull  of  a  New-born  Child  and  of  an  Adult  Man,  Drawn  as  of 
Approximately  the  Same  Size. — {Henke.) 

which  in  the  child's  skull  are  equal  in  length.  The  portion  of  the 
skull  in  front  of  a  similar  line  in  the  adult  skull  is  very  much 
greater  than  that  which  lies  behind,  the  proportion  between  the 
two  parts  being  5:3,  against  3 : 3  in  the  child  (Froriep) .  There  has, 
therefore,  been  a  decidedly  more  rapid  growth  of  the  anterior 
portion  of  the  skull,  a  growth  which  is  associated  with  a  cor- 
responding increase  in  the  dorso-ventral  dimensions  of  the  maxillae. 
These  bones,  indeed,  play  a  very  important  part  in  determining 
proportions  of  the  skull  at  different  periods.  They  are  so  the 
intimately  associated  with  the  cranial  portions  of  the  skull  that 
their  increase  necessitates  a  corresponding  increase  in  the  anterior 
part  of  the  cranium,  and  their  increase  in  this  direction  standsjin 
relation  to  the  development  of  the  teeth,  the  eight  teeth  which  are 
developed  in  each  maxilla  (including  the  premaxilla)  in  the  adult  re- 


POST-NATAL   DEVELOPMENT  485 

quiring  a  longer  bone  than  do  the  five  teeth  of  the  primary  dentition, 
these  again  requiring  a  greater  length  when  completely  developed 
than  they  do  in  their  immature  condition  in  the  new-born  child. 

But  far  more  striking  than  the  difference  just  described  is  that 
in  the  relative  height  of  the  cranial  and  facial  regions  (Fig.  289). 
It  has  been  estimated  that  the  volumes  of  the  two  portions  have  a 
ratio  of  8  : 1  in  the  new-born  child,  4  :  i  at  five  years  of  age,  and 
2:1  in  the  adult  skull  (Froriep),  and  these  differences  are  due 
principally  to  changes  in  the  vertical  dimensions  of  the  maxillae. 
As  with  the  increase  in  length,  the  increase  now  under  consideration 
is,  to  a  certain  extent  at  least,  associated  with  the  development  of 
the  teeth,  these  structures  calling  into  existence  the  alveolar  proc- 
esses which  are  practically  wanting  in  the  child  at  birth.  But  a 
more  important  factor  is  the  development  of  the  maxillary  sin- 
uses, the  practically  solid  bodies  of  the  maxillae  becoming  trans- 
formed into  hollow  shells.  These  cavities,  together  with  the 
sinuses  of  the  sphenoid  and  frontal  bones,  which  are  also  post-natal 
developments,  seem  to  stand  in  relation  to  the  increase  in  length  of 
the  anterior  portion  of  the  skull,  serving  to  diminish  the  weight  of 
the  portion  of  the  skull  in  front  of  the  occipital  condyles  and  so 
relieving  the  muscles  of  the  neck  of  a  considerable  strain  to  which 
they  would  otherwise  be  subjected. 

These  changes  in  the  proportions  of  the  skull  have,  of  course, 
much  to  do  with  the  changes  in  the  general  proportions  of  the  face. 
But  the  changes  which  take  place  in  the  mandible  are  also  impor- 
tant in  this  connection,  and  are  similar  to  those  of  the  maxillae  in 
being  associated  with  the  development  of  the  teeth.  In  the  new- 
born child  the  horizontal  ramus  is  proportionately  shorter  than  in 
the  adult,  while  the  vertical  ramus  is  very  short  and  joins  the 
horizontal  one  at  an  obtuse  angle.  The  development  of  the  teeth 
of  the  primary  dentition,  and  later  of  the  three  molars,  necessi- 
tates an  elongation  of  the  horizontal  ramus  equivalent  to  that 
occurring  in  the  maxillae,  and,  at  the  same  time,  the  separation 
of  the  alveolar  borders  of  the  two  bones  requires  an  elongation 
of  the  vertical  ramus  if  the  condyle  is  to  preserve  its  contact 
with  the  mandibular  fossa,  and  this,  again,  demands  a  diminu- 


486 


POST-NATAL   DEVELOPMENT 


tion  of  the  angle  at  which  the  rami  join  if  the  teeth  of  the  two 
jaws  are  to  be  in  proper  apposition. 

In  the  bones  of  the  appendicular  skeleton  secondary  epiphy- 
sial centers  play  an  important  part  in  the  ossification,  and  in  few 
are  these  centers  developed  prior  to  birth,  while  the  union  of  the 
epiphyses  to  the  main  portions  of  the  bones  take  place  only  to- 
ward maturity.  The  dates  at  which  the  various  primary  and  sec- 
ondary centers  appear,  and  the  time  at  which  they  unite,  may  be 
seen  from  the  following  table: 


UPPER  EXTREMITY 


Rone 

Appearance  of 

Appearance  of  Secondary 

Fusions  of 

jDone 

Primary  Center 

Centers 

Centers 

Clavide 

6lh  week. 

(At  sternal  end)  17th  year. 

20th  year. 

Scapula. 

Body 

Sth  week.          ■! 

2  acromial  isth  year. 

2  on  vertical  border  i6th  year. 

>  20th  year. 

Coracoid 

ist  year. 

I  Sth  year. 

Head  ist  year. 

Great  tuberosity  3d  year. 

Lesser  tuberosity  sth  year. 

20th  year. 

Humerus 

yth  week. 

Inner  condyle  sth  year. 

I  Sth  year. 

Capitellum  3d  year. 

Trochlea  loth  year. 

1 7th  year. 

Outer  condyle  14th  year. 

Ulna 

yth  week. 

Olecranon  loth  year 

i6th  year. 

Distal  epiphysis  4th  year. 

iSth  year. 

Radius 

yih  week. 

Proximal  epiphysis  5  th  year. 

17th  year. 

Distal  epiphysis  2d  year 

20th  year. 

Capitatum. . . . 

I  St  year. 

Hamatum 

2d  year. 

Triquetrum  . . . 

3d  year. 

Lunatum 

4th  year. 

Multangulum 

Sth  year. 

majus. 

Navicular 

6th  year. 

Multangulum 

Sth  year. 

minus. 

Pisiform 

12  th  year. 

Metacarpals  . . 

3d  year. 

20th  year. 

Phalanges 

gth-iith  week. 

3d-sth  years. 

1 7th- 1  Sth  years. 

The  dates  in  italics  are  before  birth. 


POST-NATAL   DEVELOPMENT 

487 

LOWER  EXTREMITY 

Bene 

Appearance  of 

Appearance  of  Seconadry 

Fusion  of 

Pnmary  Centei 

Centers 

Centers    ~ 

Ilium 

gth  week. 
4th  month. 

Crest  I  sth  year. 

Ischium 

Anterior  inferior  spine  15  th  year. 
Tuberosity  15  th  year. 

2 2d  year. 

Pubis 

4th  month. 

Crest  i8th  vear. 

Patella 

Cartilage  appears  at  4th  month,  ossification  in  3d  year. 

Head  ist  year. 

20th  year. 

Femur 

yth  week. 

Great  trochanter  4th  year. 

19th  year. 

Lesser  trochanter  I3th-i4th  year 

I  Sth  year. 

Condyle  gth  month 

2 1  st  year. 

Tibia 

jth  week. 

Head  end  of  gth  month. 
Distal  end  2d  year. 

2ist-2Sth  year. 

I  Sth  year. 

Fibula 

Sth  week. 

Upper  epiphysis  5  th  year. 

2  ist  year. 

Lower  epiphysis  2d  year. 

20th  year. 

Talus 

7th  month. 

Calcaneus 

6th  month. 

loth  year. 

1 6th  year. 

Cuboid 

A  few  days  after 
birth. 

Navicular 

4th  year. 

Cuneiforms  . .  . 

I  St  year. 

Metatarsals .  . . 

gth  week. 

3d  year. 

20th  year. 

Phalanges 

gth-i2th  week. 

4th-8th  year« 

1 7th- 1  Sth  years 

The  dates  in  italics  are  before  biith. 


So  far  as  the  actual  changes  in  the  form  of  the  appendicular 
bones  are  concerned,  these  are  most  marked  in  the  case  of  the  lower 
limb.  The  ossa  innominata  alter  somewhat  in  their  proportions 
after  birth,  a  fact  which  may  conveniently  be  demonstrated  by  con- 
sidering the  changes  which  occur  in  the  proportions  of  the  pelvic 
diameters,  although  it  must  be  remembered  that  these  diameters 
are  greatly  influenced  by  the  development  of  the  sacral  curve. 
Taking  the  conjugate  diameter  of  the  pelvic  brim  as  a  unit  for  com- 
parison, the  antero-posterior  (dorso-ventral)  and  transverse  diame- 
ters of  the  child  and  adult  have  the  proportions  shown  in  the  table 
on  the  opposite  page  (Fehling). 

It  will  be  seen  from  this  that  the  general  form  of  the  pelvis  in 
the  new-born  child  is  that  of  a  cone,  gradually^ diminishing  in 
diameter  from  the  brim  to  the  outlet,  a  condition  very  different 


488 


POST-NATAL   DEVELOPMENT 


from  what  obtains  in  the  adult.  Furthermore,  it  is  interesting  to 
note  that  sexual  differences  in  the  form  of  the  pelvis  are  clearly 
distinguishable  at  birth;  indeed,  according  to  Fehling's  obser- 
vations, they  become  noticeable  "during  the  fourth  month  of  intra- 
uterine development. 


Diameter 

New-bom 
Female 

Adult 
Female 

New-born 
Male 

Adult 
Male 

Conjugata  vera 

I.OO 

1. 19 
0.96 
1. 01 
0.91 
0.83 

I.OO 
1.292 
1. 19 

1. 151 
I. OS 

I-I54 

I.OO 
1.20 
o.gi 
0.99 
0.78 
0  8d 

^       Transverse 

1.294 
I    18 

>.    1  Antero-Dosterior 

*>    < 

U       Transverse.  > 

I.  14 
1.07 

T       T  C-7 

*j     1  Antero-Dosterior 

O       Transverse 

The  upper  epiphysis  of  the  femur  is  entirely  unossified  at  birth 
and  consists  of  a  cartilaginous  mass,  much  broader  than  the  rather 
slender  shaft  and  possessing  a  deep  notch  upon  its  upper  surface 
(Fig.  290).  This  notch  marks  off  the  great  trochanter  from  the 
head  of  the  bone,  and  at  this  stage  of  development  there  is  no  neck, 
the  head  being  practically  sessile.  As  development  proceeds  the 
inner  upper  portion  of  the  shaft  grows  more  rapidly  than  the  outer 
portion,  carrying  the  head  away  from  the  great  trochanter  and 
forming  the  neck  of  the  bone.  The  acetabulum  is  shallower  at 
birth  than  in  the  adult  and  cannot  contain  more  than  half  the 
head  of  the  femur;  consequently  the  articular  portion  of  the  head 
is  much  less  extensive  than  in  the  adult. 

It  is  a  well-known  fact  that  the  new-born  child  habitually  holds 
the  feet  with  the  soles  directed  toward  one  another,  a  position  only 
reached  in  the  adult  with  some  difficulty,  and  associated  with  this 
supination  or  inversion  there  is  a  pronounced  extension  of  the  foot 
{i.e.,  flexion  upon  the  leg  as  usually  understood;  see  p.  104),  it  being 
difl&cult  to  flex  the  child's  foot  beyond  a  line  at  right  angles  with 


POST-NATAL   DEVELOPMENT 


489 


the  axis  of  the  leg.  These  conditions  are  due  apparently  to  the 
extensor  and  tibialis  muscles  being  relatively  shorter  and  the  oppos- 
ing muscles  relatively  longer  than  in  the  adult,  and  with  the  elon^ 
gation  or  shortening,  as  the  case  may  be,  of  the  muscles  on  the 
assumption  of  the  erect  position,  the  bones  in  the  neighborhood  of 
the  ankle-joint  come  into  new  relations  to  one  another,  the  result 
being  a  modification  of  the  form  of  the  articular  surfaces,  especially 
of  the  talus  (astragalus).  In  the  child  the  articular  cartilage  of 
the  trochlear  surface  of  this  bone  is  continued  onward  to  a  consid- 
erable extent  upon  the  neck  of  the  bone,  which  comes  into  contact 


Fig.  290. — Longitudinal  Sections  of  the  Head   of  the  Femur  of    {A)   New 
BORN  Child  and  (B)  a  Later  Stage  of  Development. 


with  the  tibia  in  the  extreme  extension  possible  in  the  child.  In 
the  adult,  however,  such  extreme  extension  being  impossible,  the 
cartilage  upon  the  neck  gradually  disappears.  The  supination  in 
the  child  brings  the  talus  in  close  contact  with  the  inner  surface  of 
the  calcaneus  and  with  the  sus tenaculum  tali;  with  the  alteration 
of  position  a  growth  of  these  portions  of  the  calcaneus  occurs,  the 
sustentaculum  becoming  higher  and  broader,  and  so  becoming  an 
obstacle  in  the  way  of  supination  in  the  adult.  At  the  same  time  a 
greater  extent  of  the  outer  surface  of  the  talus  comes  into  contact 
with  the  lateral  malleolus,  with  the  result  that  the  articular  surface 


490  LITERATURE 

is  considerably  increased  on  that  portion  of  the  bone.  Marked 
changes  in  the  form  of  the  talo-navicular  articulation  also  occur, 
but  their  consideration  would  lead  somewhat  further  than  seems 
desirable. 

LITERATURE 

C.    Aeby:  "Die   Altersverschiedenheiten    der  menschlichen  Wirbelsaule."    Archiv 

fur  Anat.  und  Physiol.,  Anat.  Abth.,  1879. 
W.  Camerer:  " Utersuchungen  iiber  Massenwachsthum  und  Langenwachsthum  der 

Kinder,"  Jahrbuchfiir  Kinder heilkunde,  xxxvi,  1893. 
H.  H.  Donaldson:  "The  Growth  of  the  Brain,"  London,  1895. 
H.  Fehling:  "Die^Form  des  Beckens  beim  Fotus  und  Neugeborenen  und  ihre  Bezie- 

hung  zu  der  beim  Erwachsenen,"  Archiv  fUr  GynakoL,  x,  1876, 
H.    Friedenthal:  "Das   Wachsthum   des   Korpergewichtes   des    Menschen   und 

anderer  Saugethiere  in  verschiedenen  Lebensaltern,"  Zeit.   allgem.    Physiol., 

DC,  1909. 
J.  A.  Hammar:  "Ueber  Gewicht,  Involution  und  Persistenz  der  Thymus  im  Post- 

fotalleben  des  Menschen,"  Archiv  jur  Anat.  und  Phys.,  Anat.  Abth.,  Supplement, 

1906. 
W.  Henke:  "Anatomic  des  Kindersalters,"  Handbuch  der  Kinder krankheiten  (Ger- 

hardt),  Tubingen,  1881. 
C.  Hennig:  "Das  kindliche  Becken,"  Archiv  fUr  Anat.  und  Physiol.,  Anat.  Abth., 

1880. 
C.  Huter:  "Anatomische  Studien  an  den  Extremitatengelenken  Neugeborener 

und  Erwachsener,"  Archiv  fiir  patholog.  Anat.  und  Physiol.,  xxv,  1862. 
W.  Stephenson:  "On  the  Relation  of  Weight  to  Height  and  the  Rate  of  Growth  in 

Man,"  The  Lancet,  11,  1888. 
R.  Thoma:  "  Untersuchungen  iiber  die  Grosse  und  das  Gewicht  der  anatomischen 

Bestandtheile  des  menschlichen  Korpers,"  Leipzig,  1882. 
H.  Vierordt:  "Anatomische,  Physiologische  und  Physikalische  Daten  und  Tabel- 

len,"  Jena,  1893. 
H.   Welcker:  "Untersuchungen   iiber   Wachsthum  und   Bau   des    menschlichen 

Schadels,"  Leipzig,  1862. 


INDEX 


After-birth,  140 
After-brain,  390 
Age  of  embryos,  105 
Agger,  nasi,  179 
Allantois,  112,  116,  364 
Alveolo-lingual  glands,  295 

groove,  291 
Amitotic  division,  7 
Amnion,  iii,  112 
Amniotic  cavity,  57 
Amphiarthrosis,  190 
Amphiaster,  5 
Angioblast,  222 
Annulns  of  Vieussens,  235 
Anterior  commissure,  410 
Anthelix,  450 
Antitragus,  450 
Anus,  283 
Aortic  arches,  245 

bulb,  232 

septum,  237 
Archenteron,  51,  282 
Archoplasm  sphere,  4 
Arcuate  fibers,  394 
Areas  of  Langerhans,  315 
Arrectores  pilorum,  149 
Arteries,  241 

anterior  tibial,  255 

aorta,  246 

branchial,  243 

carotid,  244 

centralis  retinae,  463 

coeliac,  248 

common  iliac,  246,  252 

costo-cervical,  251 

dorsalis  pedis,  256 

epigastric,  251 

external  iliac,  249 
maxillary,  244 


Arteries,  femoral,  255 

hyaloid,  452 

hypogastric,  249,  269 

inferior  gluteal,  256 

inferior  mesenteric,  248 

innominate,  246 

intercostal,  246 

internal  mammary,  251 
maxillary,  244 
spermatic,  247 

interosseous,  252,  255 

lingual,  244 

lumbar,  246 

median,  252 

middle  sacral,  247 

peroneal,  256 

popliteal,  255 

posterior  tibial,  255 

profunda  femoris,  255 

pulmonary,  245 

radial,  254 

renal,  247 

sciatic,  255 

subclavian,  246,  252 

superficial  radial,  252 

superior  mesenteric,  248 
vesical,  249 

temporal,  244 

ulnar,  252 

umbilical,  119,  243,  248 

vertebral,  249 

vitelline,  224 
Articular  capsule,  190 
Ary-epiglottic  folds,  338 
Arytenoid  cartilages,  339 
Aster,  s 
Atresia  of  duodenum,  309 

of  pupil,  457 
Atrial  septum,  234 
Atrio-ventricular  bundle,  240 

valves,  239 


491 


492 


INDEX 


Auerbach,  plexus  of,  425 
Auricle,  449 
Axis  cylinder,  382 


B 


Bartholin,  glands  of,  367 
Belly-stalk,  72,  116 
Bile  capillaries,  311 
Bladder,  366 
Blastoderm,  45 
Blastopore,  51,  57.  59 
Blastula,  42 
Blood,  226 

islands,  223 

platelets,  230 

vessels,  222 
Body  cavity,  62 
Bone,  development  of,  156 

growth  of,  159 
Bone-marrow,  158 
Bones: 

atlas,  165,  167 

axis,  167 

carpal,  187,  190,  486 

clavicle,  185,  486 

coccyx,  168 

conch  ae,  178 

epistropheus,  165,  167 

ethmoid,  177 

femur,  189,  487,  488 

fibula,  189,  487 

frontal,  180 

humerus,  187,  486 

hyoid,  184 

ilium,  188,  487 

incus,  182,  445 

innominate,  188,  487 

interparietal,  175 

ischium,  188,  487 

lachrymal,  180 

malleus,  182,  445 

mandible,  182 

maxilla,  181 

metacarpal,  188,  486 

metatarsal,  190,  487 

nasal,  180 


Bones: 

occipital,  172,  17s 

palatine,  181 

parietal,  180 

patella,  189,  487 

periotic,  171,  179 

phalanges,  188,  190,  486,  487 

precoracoid,  191 

premaxilla,  181 

pubis,  i88,  487 

radius,  187,  486 

ribs,  164,  167 

sacrum,  168,  482 

scapula,  186,  486 

sphenoid,  176 

stapes,  446 

sternum,  168 

suprasternal,  169 

tarsal,  189,  487,  488 

temporal,  179 

tibia,  189,  487 

turbinated,  178 

ulna,  187,  486 

vertebrae,  162,  166,  483 

vomer,  178 

zygomatic,  180 
Brachia  conjunctiva,  398 
Brain,  390,  480 
Branchial  arches,  93,  99 

clefts,  93 

epithelial  bodies,  296,  297 

fistula,  94 
Branchiomeres,  84 
Bronchi,  335 

Bucconasal  membrane,  285 
Bulbo-urethral  glands,  367 
Bulbo-vestibular  glands,  367 
Burdach,  fasciculus  of,  389 
Bursa  omentalis,  327 


Caecum,  304,  307 

Calcar,  408 

Canal,  inguinal,  371 
of  Cloquet,  467 
of  Gartner,  361 
of  Nuck,  369 


INDEX 


493 


Canal  of  Petit,  467 
Canalized  fibrin,  130 
Capillaries,  224 
Cartilages  of  Santorini,  339 

of  Wrisberg,  339 
Caruncula  lacrimalis,  472 
Cauda  equina,  388 
Caul,  115 
Cell,  I,  3 

division,  s 

theory,  i 
Centrosome,  4 
Cerebellum,  396 
Cerebral  aqueduct,  399 

convolutions,  407 

cortex,  412 

hemispheres,  404 

peduncles,  398,  399 
Charcot,  crystalloid  of,  14 
Cheek  groove,  293 
Chin  ridge,  103 
Chondrioconts,  5 
Chondriosomes,  $ 
Chondrocranium,  172,  175 
Chorda  canal,  60 

dorsalis,  78 

endoderm,  78 
Chorioid  coat,  454,  467 

plexus,  393,  401,  406 
Chorioidal  fissure  of  brain,  406 

of  eye,  452,  457 
Chorion,  71,  121 

frondosum,  127 

laeve,  127 
Chorionic  villi,  1 26 
Chromaflfine  organs,  374 
Chromatin,  4 
Chromosomes,  5 

reduction  of,  15,  30 
Ciliary  body,  458 

ganglion,  429 

muscle,  469 
Cleft  palate,  286 

sternum,  171 
Clitoris,  367 
Cloaca,  282,  363 
Cloacal  membrane,  283 


Cloquet,  canal  of,  467 
Coccygeal  ganglion,  276 
Ccelom,  51  62,  73,  81 
Collateral  eminence,  409 
Colliculus  seminalis,  362 
Coloboma,  458 
Colon,  306 
Colostrum,  153 
Conjunctiva,  469 
Connective  tissues,  155 
Cornea,  453,  468 
Corniculate  cartilages,  338 
Corona  radiata,  21,  357 
Coronary  sinus^  234,  260 
Corpora  mammillaria,  402 

quadrigemina,  399 
Corpus  albicans,  24 

callosum,  410 

luteum,  23 

striatum,  404 
Corti,  spiral  organ  of,  441 
Cowper,  glands  of,  367 
Cranial  nerves,  414 

sinuses,  257 
Cricoid  cartilage,  339 
Cuneiform  cartilages,  338 
Cutis  plate,  83 
Cytoplasm,  3 
Cyto-trophoblast,  57,  125 

D 

Darwin's  tubercle,  450 
Decidua  basalis,  132,  135 

capsularis,  124,  133,  134 

reflexa,  124,  133 

serotina,  132 

vera,  132,  133 
Decidual  cells,  134,  140 
'Dendrites,  382 
Dental  groove,  287 

papilla,  287 

shelf,  287 
Dentate  gyrus,  408 
Dermatome,  83 
Descent  of  ovary,  369 

of  testis,  370 


494 


INDEX 


Diaphragm,  323 
Diarthrosis,  191 
Diencephalon,  390,  399 
Discus  proligerus,  20,  357 
Double  monsters,  49 
Duct  of  Santo rini,  315 

of  Wrisberg,  315 
Ductus  arteriosus,  245,  269 
Botalli,  245 
choledochus,  310 
cochlearis,  439 
Cuvieri,  259 
ejaculatorius,  359 
endolymphaticus,  437 
reuniens,  439 
venosus,  262 
Duodenum,  305,  306,  309 

E 
Ear,  436 

Ebner,  glands  of,  436 
Ectoderm,  51 
Embryo,  age  of,  105 

external  form,  89 

growth  of,  477 
Embryonic  disc,  57 
Embryo  troph,  125 
Enamel  organ,  287,  289 
Enchylema,  4 
Endocardium,  230 
Endoderm,  51,  54,  57 
Enveloping  layer,  45,  48 
Ependymal  cells,  381 
Epiblast,  51 

Epibranchial  placodes,  422 
Epidermis,  143 
Epididymis,  358 
Epiglottis,  338 
Epiphyses,  158 
Epiphysis  cerebri,  400 
Epiploic  foramen,  327 
Episternal  cartilages,  169 
Epitrichium,  143 
Eponychium,  147 
Epoophoron,  360 
Erythrocytes,  226,  227 
Erythroplastids,  227 


Eustachian  tube,  296,  444 

valve,  23s 
Extrauterine  pregnancy,  23 
Eye,  451 
Eyelids,  469 


Fallopian  tubes,  361 
Fasciculus  communis,  419 

of  Burdach,  389 

of  Goll,  388 

solitarius,  393,  419 
Fenestra  cochleae,  444 

ovalis,  444 

rotunda,  444 

vestibuli,  444 
Fertilization  of  ovum,  31 
Fetal  circulation,  267 
Fibrinoid,  130 
Fifth  ventricle,  411 
Filum  terminale,  388 
Fimbria,  410 

ovarica,  362 
Foliate  papillae,  436 
Fontana,  spaces  of,  469 
Foramen  caecum,  298 

of  Winslow,  327 

ovale,  234,  241 
Fore-brain,  390 
Formatio  reticularis,  394 
Fornix,  410 
Frontal  sinuses,  178 
Funiculus  cuneatus,  389 

gracilis,  388 
Furcula,  296,  337 


Gartner,  canals  of,  361 
Gall  bladder,  310 
Ganglionated  cord,  427 
Gastral  mesoderm,  53,  61 
Gastrula,  51 
Geniculate  bodies,  401 
Genital  folds,  367 
ridge,  341,  353 


INDEX 


495 


Genital  swelling,  367 

tubercle,  367 
Germ  cells,  8 

layers,  50,  63 

plasm,  8 
Giraldes,  organ  of,  358 
Glands  of  Bartholin,  367 

bulbo-urethral,  367 

bulbo-vestibular,  367 

of  Cowper,  367 

of  Ebner,  436 

Meibomian,  470 

of  Moll,  470 

salivary,  293 

tarsal,  470 
Goll,  fasciculus  of,  388 
Graafian  follicle,  19 
Great  omentum,  327 
Groove  of  Rosenmiiller,  297 
Gubernaculum  testis,  360 
Gynsecomastia,  153 


Haemoblasts,  222 
Haematopoietic  organs,  226 
Haemolymph  nodes,  275 
Hairs,  148 
Hare  lip,  102,  182 
Hassall's  corpuscles,  300 
Haversian  canals,  160 
Head  cavities,  82 

process,  59,  72 
Heart,  230,  480 
Helix,  450 
Hensen's  node,  59 
Hermaphroditism,  369 
Hind-brain,  390 
Hippocampus,  408 
Hyaloid  canal,  467 
Hydatid  of  Morgagni,  359 

stalked,  362 
Hydramnios,  115 
Hymen,  362 
Hyperthelia,  152 
Hypertrichosis,  150 
Hypoblast,  51 
Hypochordal  bar,  163 


Hypophysis,  403 
Hypospadias,  369 
Hypothalamic  region,  401 


Idiochromosomes,  16,  34 

Iliac  lymph  sac,  270 

Implantation  of  ovum,  121 

Infracardial  bursa,  327 

Infundibulum,  404 

Inguinal  canal,  371 

Inner  cell  mass,  48 

Insula,  409 

Interarticular  cartilages,  191 

Intercarotid  ganglion,  377 

Intermediate  cell  mass,  80 

Interrenal  organs,  374 

Interventricular  foramen,  405 

Intervertebral  fibro-cartilage,  162,  164 

Intestine,  304,  481 

Iris,  458 

Isthmus  cerebri,  390,  398 


Jacobson,  organ  of,  434 

Joints,  190 

Jugular  lymph  sac,  270 

K 

Karyokinesis,  7 
Karyoplasm,  3 
Kidney  (see  Metanephros),  347,  48c 

L 

Labia  majora,  368 

minora,  368 
Lachrymal  gland,  471 
Lamina  terminalis,  402 
Langerhans,  areas  of,  315 
Langhans  cells,  129 
Lanugo,  149 
Larynx,  337 
Lateral  thyreoids,  301 
Lens,  452,  454 


496 


INDEX  • 


Lesser  omentum,  326 

Leukocytes,  229 

Ligaments: 

broad,  of  uterus,  352,  360 
coraco-humeral,  217 
coronary,  of  liver,  324 
falciform,  of  liver,  324 
fibular  lateral,  of  knee,  201 
flavan, 164 

inguinal,  353,  360,  362 
interspinous,  164 
mucosum,  192 
of  the  ovary,  362 
pectinatum  iridis,  469 
round,  of  liver,  270 
round,  of  uterus,  362 
sacro-tuberous,  201 
spheno-mandibular,  184 
suspensory  of  lens,  466 

Limbs,  93,  103 

Lip-ridge,  103 

Lips,  286 

Liver,  309,  480 

Lubarsch,  crystalloid  of,  14 

Lungs,  334,  481 

Luschka's  ganglion,  276 

Lymphatics,  270 

Lymph  nodes,  273 
sacs,  270 

Lymphocytes,  229,  276 

M 

Magma,  cellular,  68 

reticular,  70 
Mammary  gland,  150 
Mandibular  process,  94 
Mastoid  cells,  447 
Maturation  of  ovum,  28 
Maxillary  antrum,  178 

process,  94 
Meckel's  cartilage,  173,  182 

diverticulum,  116,  307 
Mediastina,  325 
Medulla  oblongata,  390 
Medullary  canal,  77,  90 

cords,  356 


Medullary  folds,  75 

groove,  73 

sheath,  386 
Megacaryocytes,  230 
Meibomian  glands,  470 
Meissner,  plexus  of,  425 
Membrana  pupillaris,  457 

reuniens,  84 

tectoria,  442 
Membrane  bone,  156 
Menstruation,  26 
Mesencephalon,  390,  398 
Mesenchyme,  64 
Mesenteriole,  330 
Mesentery,  326 
Mesocardium,  319 
Mesocolon,  328 
Mesoderm,  51 

somatic,  80 

splanchnic,  81 

ventral,  80 
Mesodermic  somites,  75,  79 
Mesogastrium,  326 
Mesonephros,  345,  358 
Mesorchium,  359,  371 
Mesothelium,  64 
Metamere,  86 
Metanephros,  347 
Metencephalon,  390,  395 
Mid-brain,  390 
Middle  ear,  444 
Milk  ridge,  150 
Mitochondria,  5 
Mitosis,  7 

Moll,  glands  of,  470 
Montgomery's  glands,  151 
Morgagni,  hydatid  of,  359 
Morula,  46 
Mouth  cavity,  285 
Miillerian  duct,  351 
Muscle  plates,  83 
Muscles: 

arrectores  pilorum,  149 

biceps  femoris,  217 

branchiomeric,  207 

chondroglossus,  211 

ciliary,  469 


INDEX 


497 


Muscles: 

coccygeus,  206 

constrictor  of  pharynx,  209,  301 

cranial,  206 

curvator  coccygis,  206 

depressors  of  hyoid,  203 

digastric,  209 

dilatator  iridis,  459 

dorsal,  202 

eye,  207 

facial,  209 

gastrocnemius,  215,  219 

geniohyoid,  203 

genioglossus,  203 

glosso-palatinus,  209 

hyoglossus,  204 

hyposkeletal,  204 

intercostal,  204 

laryngeal,  209,  340 

latissimus  dorsi,  200,  216 

levator  ani,  206 

limb,  211 

longus  capitis,  204 

colli,  204 
lumbrical,  219 
masseter,  209 
mylohyoid,  209 
obliqui  abdominis,  204 
occipito-f  rental  is,  201,  209 
omohyoid,  200 
pectorals,  216 
perineal,  206 
peroneus  longus,  217 
platysma,  209 
pronator  quadratus,  217 
psoas,  204 
pterygoids,  209 
pyramidalis,  203 
rectus  abdominis,  201,  203 
sacro-spinalis,  201,  204 
scaleni,  204 
serrati  posteriores,  201 
serratus  anterior,  201,  216 
skeletal,  199 
soleus,  215,  219 
sphincter  ani,  206 
sphincter  cloacae,  206 
32 


Muscles: 

sphincter  cloacae,  206 
iridis,  459 

stapedius,  209,  446 

sternohyoid,  200 

sterno mastoid,  200,  204,  211 

styloglossus,  204 

stylohyoid,  209 

stylopharyngeus,  209,  301 

temporal,  209 

tensor  tympani,  209,  445 
veli  palati,  209 

transversus  abdominis,  204 
thoracis,  204 

trapezius,  200,  204,  211 
Muscle  tissue,  195 
Myelencephalon,  390,  393 
Myelin,  386 
Myelocytes,  229 
Myoblasts,  197 
Myocardium,  230 
Myotome,  83,  199 

N 

Nails,  14s 
Nape  bend,  93 
Nasal  pit,  10 1 

process,  loi 
Naso-lachrymal  duct,  471 
Nephrogenic  cord,  346 
Nephrostome,  344 
Nephrotome,  83 
Nerve  components,  415,  418,  421 

roots,  384 
Nerves: 

auditory,  420 

cranial,  414 

hypoglossal,  417 

olfactory,  433 

optic,  462 

recurrent,  340 

spinal,  413 

accessory,  421 

splanchnic,  429 

terminal,  417 
Nerve  tissue,  381 
Neural  crest,  384 


498 


INDEX 


Neurenteric  canal,  6i,  73,  76 
Neuroblasts,  382 
Neuroglia  cells,  382 
Neuromeres,  423 
Neurone  theory,  386 
Nitabuch's  stria,  139 
Non-sexual  reproduction,  9 
Normoblasts,  227 
Notochord,  77 
Nuck,  canal  of,  369 
Nucleoli,  4 
Nucleus,  3 


Odontoblasts,  290 

(Esophagus,  301 

(Estrus,  28 

Olfactory  lobes,  41 1 
organ,  433 

Olivary  body,  394 

Omentum,  327 

Oocyte,  29 

Optic  cup,  452,  457 
recess,  402 

Oral  fossa,  91,  loi,  282 

Organ  of  Giraldes,  358 
of  Jacobson,  434 
of  Rosenmiiller,  360 

Organs,  3 

chromafline,  374 
interrenal,  374 
of  taste,  435 
of  Zuckerkandl,  378 
suprarenal,  374,  480 

Osteoblasts,  156 

Osteoclasts,  160 

Otocyst,  437 

Otic  ganglion,  429 

Ovary,  356 

descent  of,  369 

Ovulation,  22,  26 

Ovum,  19 

fertilization  of,  31 
implantation  of,  121 
maturation  of,  28 
segmentation  of,  41 


Palate,  285 
Pancreas,  314,  481 
Paradidymis,  358 
Paraphysis,  400 
Parathymus,  301 
Parathyreoid  bodies,  299 
Paroophoron,  360 
Parotid  gland,  293 
Parovarium,  360 
Parthenogenesis,  9 
Penis,  368 

Pericardial  cavity,  320,  321 
Perineal  body,  366 
Perionyx,  147 
Periosteum,  157 
Periotic  capsule,  171,  178 
Peritoneum,  326 
Petit,  canal  of,  467 
Pfliiger's  cords,  356 
Pharyngeal  bursa,  296 

membrane,  282 

tonsil,  296 
Pharynx,  296 

Pharyngo-palatine  arches,  285 
Pineal  body,  400 
Pinna,  450 
Pituitary  body,  403 
Placenta,  135,  140 

accessory,  127 

deciduate,  140 

embryotrophic,  126 

haematrophic,  126 

indeciduate,  140 

praevia,  136 
Placentar  infarcts,  139 
Plasmodi-trophoblast,  57,  125 
Plasmodium,  125 
Pleurae,  325 

Pleuro-peritoneal  cavity,  81,  322 
Plica  semilunaris,  470 
Polar  globules,  30 
Polycaryocytes,  230 
Polymastia,  152 
Polyspermy,  35 
Pons,  395 

flexure,  392 


INDEX 


499 


Post-anal  gut,  283 
Post-natal  development,  466 
Precaudal  recess,  283 
Precoracoid,  191 
Prepuce,  368 
Primitive  groove,  59,  72 

streak,  53,  72 
Processus  globularis,  loi 
Pronephric  duct,  342 
Pronephros,  342 
Pronuclei,  32 
Prooestrum,  28 
Prostate  gland,  365 
Prostomial  mesoderm,  53,  61 
Protoplasm,  2 
Pro  to  vertebrae,  80 


Rathke's  pouch,  287,  403 

Rauber's  covering  layer,  48 

Rectum,  283 

Red  nucleus,  399 

Reduction  of  chromosomes,  15,  30 

Restiform  body,  395 

Rete  cords,  353 

ovarii,  357 

testis,  356 
Retina,  460 

Retroperitoneal  lymph  sac,  270 
Rhinencephalon,  412 
RosenmtiUer,  groove  of,  297 

organ  of,  360 


Sacculus,  439 
Sacral  bend,  93 
Salivary  glands,  293 
Santorini,  cartilages  of,  339 

duct  of,  315 
Sarcode,  i 
Scala  tympani,  444 

vestibuli,  443 
Sclerotic  coat,  454,  467 
Sclerotome,  83 
Scrotum,  368 
Sebaceous  glands,  149 


Segmentation  nucleus,  32 

of  ovum,  41 
Semicircular  ducts,  438 
Semilunar  valves,  240 
Seminiferous  tubules,  356 
Septum  aortic,  237 

pellucidum,  411 

primum,  234 

secundum,  234 

spurium,  234 

transversum,  321,  323,  327 

ventricular,  237 
Sertoli  cells,  14 
Sex  cells,  353 

cords,  353 
Sexual  reproduction,  9 
Sinusoid,  225 
Sinus,  coronary,  234,  260 

pocularis,  359 

praecervicalis,  100 

terminalis,  224 

venosus,  232 
Situs  inversus  viscerum,  49 
Skin,  143,  481 
Skull,  171,  483 
Socia  parotidis,  293 
Solitary  fasciculus,  393,  419 
Somatic  cells,  8 
Spaces  of  Fontana,  469 
Spermatic  cord,  371 
Spermatid,  14 
Spermatocyte,  14 
Spermatogenesis,  13 
Spermatogonia,  14 
Spermatozoon,  11 
Sphenoidal  cells,  178 
Spheno-palatine  ganglion,  429 
Spinal  cord,  387,  480 

nerves,  413 
Spiral  organ  of  Corti,  441 
Spleen,  275,  480 
Stomach,  302 
Sublingual  ganglion,  429 

gland,  295 
Submaxillary  ganglion,  429 
Submaxillary  gland,  294 
Substance  islands,  223 


500 


INDEX 


Sudoriparous  glands,  150 
Sulcus  Monroi,  399 
Superfetation,  37 
Suprabranchial  placodes,  422 
Suprarenal  bodies,  374,  480 

accessory,  376 
Supratonsillar  fossa,  297 
Suture,  190 

Sympathetic  nervous  system,  423 
Synchondrosis,  190 
Syncytium,  125 
Systems,  3 


Tail  filament,  96 
Tarsal  glands,  470 
Taste,  organs  of,  435 
Teeth,  287 
Tegmentum,  398 
Telencephalon,  390,  402 
Testis,  354 

descent  of,  369 
Thalami,  401 
Thebesian  valve,  235 
Thoracic  duct,  272 
Thymus  gland,  299,  480 
Thyreoid  cartilage,  338 

gland,  298,  480 
Thyreo-glossal  duct,  298 
Tissues,  3 
Tongue,  291 
Tonsils,  297 
Touch,  organs  of,  435 
Trachea,  337 
Tragus,  450 
Trophoblast,  57 
Tuba  auditiva,  444 
Tubae  uterinae,  361 
Tuber  cinereum,  402 
Tuberculum  impar,  291 
Tubuli  recti,  356 
Tunica  albuginea,  354,  356 

vaginalis  testis,  371 

vasculosa  lentis,  457 
Tween-brain,  390 
Twin-development,  48 


Tympanic  cavity,  444 
membrane,  448 

U 

Ultimo-branchial  bodies,  301 
Umbilical  cord,  95,  119 
Umbilicus,  90 
Urachus,  118,  364 
Ureter,  348 
Urethra,  365 
Urogenital  sinus,  364 
Uterovaginal  camil,  353 
Uterus,  361,  363 

masculinus,  359 
Utriculus,  439 

prostaticus,  359 


Vagina,  361,  363 
Vaginal  process,  369 
Vallate  papillae,  435 
Vas  deferens,  359 
Veins: 

anterior  cardinal,  256 
tibial,  267 

ascending  lumbar,  266 

azygos,  266 

basilic,  267 

cephalic,  266 

emissary,  260 

external  jugular,  260 

hemiazygos,  266 

hepatic,  263 

inferior  vena  cava,  265 

innominate,  259 

internal  jugular,  256 

jugulo-cephalic,  267 

limb,  266 

long  saphenous,  267 

portal,  262 

posterior  cardinal,  256,  260 

primary  fibular,  267 
ulnar,  266 

pulmonary,  235,  267 

renal,  265 


INDEX 


501 


Veins : 

subcardinal,  263 

superior  vena  cava,  259 

supracardinal,  265 

suprarenal,  265 

umbilical,  121,  262 

vitelline,  224,  261 
Velum,  anterior,  398 

interpositum,  400 

marginal,  381 

posterior,  393 
Vena  capitis,  256 
Ventricular  septum,  237 
Vermiform  appendix,  307 
Vernix  caseosa,  115,  149 
Vertex  bend,  90 
Vesicula  seminalis,  359 
Vieussens,  annulus  of,  235 
Villi,  chorionic,  126 

intestinal,  308 


Vitreous  humor,  454,  465 
Vulva,  368 

W 

Wharton's  jelly,  121 
Winslow,  foramen  of,  327 
Wirsung,  duct  of,  315 
Witch  milk,  153 
Wolfl&an  body,  342,  358 

duct,  342,  358 

ridge,  341 
Wrisberg,  cartilage  of,  339 


Yolk  sac,  89,  no,  115 
stalk,  89,  93,  no 


Zona  pellucida,  21 
Zuckerkandl,  organ  of,  378 


DATE    DUE    SLIP 

UNIVERSITY  OF  CALIFORNIA  MEDICAL  SCHOOL  LIBRARY 

THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


Tlw.  ;n,  i<i:i  J 


k  ■  I 


FEB  ^ 
APR  1  4  19261 

^'^y     7  1826. 


SEP  18  1923 
APR  SO  1924 

JUN  1  9  1924 


m  1 7  ^923 
APR  8  -  1930 

FFB  6      1931 
UlAR    19  1932 

i943 
DEC  1 4  1943 

DEC  i  2  195? 


2m-12,'19 


0!.I601      HCi 
I!16 


8058 


urrich,   J. P. 
Ill©  develorment  of 


Ijie  nmnan  t 
f^.anual   of 


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Gth  ed. 


DVlTOlO^r* 


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JAN  28  ]mA&.^^i225 


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Library  of  the 
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